I had two issues related to this. First, I had to go to the registry editor and enable the add in to start on load (each time I opened Excel, it was again inactive): HKCU\Software\Microsoft\Office\Excel\Addins\ [ YOUR ADD-IN ]\LoadBehavior and set that REG_DWORD to a 3.
Then, I noticed that there is an apparent bug in PAD launch Excel, because even if I enable the advanced slider to "Load add-ins and macros", it does not. So, the solution for now is to "Run Application" to get Excel running and then "Attach to running Excel" to get a handle to that workbook.
Hope it helps.