In vbscript you don't have direct access to the excel enumerations, so xlUp can't be used. Try its value -4162 instead:
ExcelWB.Activesheet.Range("A65536").End(-4162).Row to find the last row in column A.
Edit: There's a list of enumerations on msdn. Or you can look them up in the object browser of the vba ide of excel.