It could be caused by an update applied to the 500 client, Office or Windows in my experience. Verify that dll is in  C:\Program Files (x86)\Common Files\BIExcelFunctions\32bit. If it isn't, you may need to reinstall Sage Intelligence, if it is then try:

1. In Excel go to File / Options.
2. Click Add Ins, select the COM Add-Ins and Go.
3. Verify the report designer is selected. If it is already checked then deselect it and OK then reopen the COM Add-Ins and select it again (you might try closing Excel between select or deselect).
4. If the report designer add-in does not appear in the list, add it by clicking on the Add button, then find it at C:\Program Files (x86)\Common Files\BIExcelFunctions\32bit\Sage.BI.ExcelFunctions.dll.

Are you saying you can't add the add-ins? Have not seen that issue, but when want to make sure you've attempted to confirm/re-add the add-ins?

If it is not there then hit the browse button. Go to this directory depending upon the bit version of excel. Double click the file in the appropriate bit folder.

Darrick