It may very well be that whatever was working in Excel 2000 is not supported in 2007. Microsoft's policy is to support the current version (2010) and one version before that (i.e. 2007).
If the company that provided the add-in for Excel 2000 has folded since, I'm afraid you're on pretty thin ice.
Presuming that the add-in was VBA code, it is very probable that the code does not work as expected with Excel 2007. Although there is a certain degree of backwards compatibility, Excel 2007 introduced the ribbon, and access to add-ins had changed a big deal.
You could try to locate the add-in via the Excel Options (found under the Office Globe) - Add-ins - Manage - Go button - browse to the folder where the Add-in is stored and then select and install it this way.
If that does not work, you probably need to look for an alternative to the add-in.
cheers, teylyn