Support informed us that the Add-Ins are not compatible with the 64-bit versions of Office.
See R&D's response below...
''Unfortunately 64bit Office and won't be supported in this version of Quality Center. The problem is the technical architecture. Since Excel add-in is running on top OTA API which is 32 bit it is not possible to work with the addin in a 64bit version of Office. We can submit an ER to request a 64bit version of the QC Client but know that it wouldn't affect this current iteration of Quality Center; it would be in a future version. In order to use the addin a 32bit version of Office would have to be used.''