I finally found the solution ...
Right after creating the TimelinerDataSource (Dim dsTl As New Tl.TimelinerDataSource([Name])), I call this procedure to create the fields in the list and assign them (equivalent to the Form "Field Selector")
Public Sub DataSource_CreaCampos(ByRef ds As Tl.TimelinerDataSource)
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("UniqueID", "ID exclusivo"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("ID", "ID"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Start", "Inicio"))
ds.TaskActualStartField = ds.AvailableFields.Last
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("ActualStart", "Inicio real"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("EarlyStart", "Inicio anterior"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("LateStart", "Inicio posterior"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Start1", "Inicio 1"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Start2", "Inicio 2"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Start3", "Inicio 3"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Start4", "Inicio 4"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Start5", "Inicio 5"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Start6", "Inicio 6"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Start7", "Inicio 7"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Start8", "Inicio 8"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Start9", "Inicio 9"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Start10", "Inicio 10"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("BaselineStart", "Inicio de línea base"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline1Start", "Inicio de línea base 1"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline2Start", "Inicio de línea base 2"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline3Start", "Inicio de línea base 3"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline4Start", "Inicio de línea base 4"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline5Start", "Inicio de línea base 5"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Finish", "Finalización"))
ds.TaskActualEndField = ds.AvailableFields.Last
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("ActualFinish", "Fin real"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("EarlyFinish", "Fin anterior"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("LateFinish", "Fin posterior"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Finish1", "Fin 1"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Finish2", "Fin 2"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Finish3", "Fin 3"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Finish4", "Fin 4"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Finish5", "Fin 5"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Finish6", "Fin 6"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Finish7", "Fin 7"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Finish8", "Fin 8"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Finish9", "Fin 9"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Finish10", "Fin 10"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("BaselineFinish", "Fin de línea base"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline1Finish", "Fin de línea base 1"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline2Finish", "Fin de línea base 2"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline3Finish", "Fin de línea base 3"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline4Finish", "Fin de línea base 4"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline5Finish", "Fin de línea base 5"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("ActualCost", "Coste real"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("BaselineCost", "Coste de línea base"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline1Cost", "Coste de línea base 1"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline2Cost", "Coste de línea base 2"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline3Cost", "Coste de línea base 3"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline4Cost", "Coste de línea base 4"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline5Cost", "Coste de línea base 5"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline6Cost", "Coste de línea base 6"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline7Cost", "Coste de línea base 7"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline8Cost", "Coste de línea base 8"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline9Cost", "Coste de línea base 9"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Baseline10Cost", "Coste de línea base 10"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Cost", "Coste"))
ds.TaskMaterialCostField = ds.AvailableFields.Last
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Cost1", "Coste 1"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Cost2", "Coste 2"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Cost3", "Coste 3"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Cost4", "Coste 4"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Cost5", "Coste 5"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Cost6", "Coste 6"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Cost7", "Coste 7"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Cost8", "Coste 8"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Cost9", "Coste 9"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Cost10", "Coste 10"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("FixedCost", "Coste fijo"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("RemainingCost", "Coste restante"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Text1", "Texto 1"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Text2", "Texto 2"))
ds.SetUserFieldByIndex(0, ds.AvailableFields.Last)
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Text3", "Texto 3"))
ds.SetUserFieldByIndex(1, ds.AvailableFields.Last)
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Text4", "Texto 4"))
ds.SetUserFieldByIndex(2, ds.AvailableFields.Last)
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Text5", "Texto 5"))
ds.SetUserFieldByIndex(3, ds.AvailableFields.Last)
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Text6", "Texto 6"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Text7", "Texto 7"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Text8", "Texto 8"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Text9", "Texto 9"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Text10", "Texto 10"))
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Text11", "Texto 11"))
ds.TaskTypeField = ds.AvailableFields.Last
ds.AvailableFields.Add(New Tl.TimelinerDataSourceField("Text12", "Texto 12"))
End Sub