To start with, have both Office and Excell been updated with all available updates (if you don't have the Office update checker running automatically, you can launch it from the Help menu of Excel or any other Office app).
Second, do you get this error when just opening Excel directly, or only when opening some particular .xls files? If the latter, I'm going to guess they have some add-on feature that isn't supported on the Mac. If the former, we'll go from there.