ActiveDocument object not recognized

A beginner's question:

I am trying to insert a column into col position 2 in a table, which apparently is to be done with the following code:

ActiveDocument.Tables(1).Columns(1).Select
Selection.InsertColumnsRight

However, when I run the macro, the object "Activedocument" is empty.
What is wrong here?
(Excell 2003 running under Windows XP)

Thanks!