Hi David,
First of all, I would recommend to verify that you have installed the ridht version of EPM Add in, if you have Excel 32 bits then you have to install normal version of EPM Addin, if you have Excel 64 bits then you have to install the BIG VOLUME version of EPM Add in. (SAP Note : 1990386 - EPM Add-in: Error launching Office Client: Object
reference not set to an instance of an object
Then try to clear metadata cash when open EPM Addin.
Also take a look at below Notes :
1828291 - BPC MS : Object reference not set to an instance of an
object
Finally if below issue persists, then try to re-install EPM to see if it helps.
Best Regards,
Dorra.