The support team can be reached at email@example.com; this forum is mostly a bunch of Tableau users, not a true "support" area. Also, they'll need to know if you're running 32 bit or 64 bit versions of Excel (and Tableau).
As a workaround, can you just save the Excel files as an earlier version (assuming there's an issue with 2013)?
There's a note about an error with 64-bit Excel 2013 here: Resolving Connection Errors | Tableau Software
What version of Office 2013 do you have installed, 32-bit or 64-bit? Tableau 8.0.x only supports 32-bit drivers. If so, you may need to install the Microsoft Office 32-bit drivers (Drivers & Activation | Tableau Software)
From that drivers page you can find PowerPivot drivers for SharePoint, which will depend on the version of SQL. I'm assuming you'll have SQL2012.