What am I missing?

I've just installed Excel 2003 and I'm trying to write a custom add-in to
format RS reports exported to Excel.

I'm using Workbook_Open to accomplish the formatting and I've saved my work
book as an Excel Add-in (.xla) in the Addin directory. The problem I'm
having is that when I make changes/additions to the code in VBA when I go to
Excel and open one of the reports with the current instance of Excel nothing
happens, it's as if Excel isn't even seeing the add-in. However, it I go out
and open up a new instance of Excel the code will execute.

I've got the lowest level of security set in Excel and I have the add-in
selected in Tools Addins.

I don't remember it being this difficult in Excel 2000 - I seem to remember
that I wrote my code and was able to test it with the same instance of
Excel.

Like I said at the top - What am I missing?

TIA,
Bill Youngman