Tip: You must use a 32-bit browser when running on a 64-bit client

This will save you a few hours of debugging. Virtually every SharePoint feature and application, e.g. Excel, PPT, and Word Web Apps, do not support 64-bit browsers. In order to work around this behavior, please use a 32-bit browser. While others have issues around compatibility, this restriction applies to PowerPivot for one HUGE reason. And that reason is (of all things, wait for it . . .): Silverlight. We use Silverlight in two big value-add areas: the various extension views in a Gallery (to display the workbook thumbnails) and as the “way-back machine” on the PowerPivot Management Dashboard (the one that allows you to go back in time to look at your usage information – the bubble chart). Thus you have to stick with a 32-bit browser even if you are running a 64-bit OS. Sorry.