When you say it works fine on your machine, I assume you also did the Excel upgrade? What other differences are there between your machine and those of the users having the problem?
Have you tried uninstalling the add-in and reinstalling it?
Do the have the newest patch loaded for v9.0? v9.0 has long been out of support, but Excel 2007 is supposed to be supported. It is possible, if you don't have the latest patch, that there may have been a defect that was corrected.