First of all you have to point to your table object inside the drawing:
For Each Object in thisdrawing.modelspace
If Object.ObjectName="AcadTable" then
Set MyTable=Object
end if
Next
.... Apply here the example for cells manipulation ...
Later you can use the online help related to Cells managing inside the drawing, removing AddTable.
Or If you want to add a Table and manipulate the cells use the entire code below coming from online help.
Sub Example_CellManipulation()
' This example adds a table in model space and sets and gets a column name
Dim MyModelSpace As AcadModelSpace
Set MyModelSpace = ThisDrawing.modelSpace
Dim pt(2) As Double
Dim MyTable As AcadTable
Dim cName As String
' Set MyTable = MyModelSpace.AddTable(pt, 5, 5, 10, 30) ' here you add Table
Call MyTable.SetCellDataType(2, 2, acLong, acUnitDistance)
Call MyTable.SetCellFormat(1, 3, "testFormat")
Call MyTable.SetCellState(4, 3, acCellStateContentLocked)
Call MyTable.SetCellValue(1, 4, 5)
MsgBox MyTable.GetCellValue(1, 4) & " is the test cell's value "
ZoomExtents
End Sub