Here's the code for appearances, to put in the Form code :

Private Sub UserForm_Initialize()
Dim oBiblis As AssetLibraries
Dim oBibli As AssetLibrary
Dim oProjectBiblis As ProjectAssetLibraries
Dim ligne(1)
'Get only Appearance libraries referenced by the current ipj
Set oBiblis = ThisApplication.DesignProjectManager.ActiveDesignProject.AppearanceLibraries
'Fill the libraries list
For Each oBibli In oBiblis
Liste_biblis.AddItem oBibli.DisplayName
Next oBibli
'Initialize the list on the 1st value
Liste_biblis.Value = Liste_biblis.List(1)
End Sub
'**************************************************
Private Sub Liste_biblis_Change()
Dim oBibli As AssetLibrary
Dim oAppearances As AssetsEnumerator
Dim oAppearance As Asset
'Get the name of the library selected in the Libraries list
Set oBibli = ThisApplication.AssetLibraries.item(Liste_biblis.Value)
'Get the appearances
Set oAppearances = oBibli.AppearanceAssets
'Fill the list
Liste_apparences.Clear
For Each oAppearance In oAppearances
Liste_apparences.AddItem oAppearance.DisplayName
Next oAppearance
End Sub