PowerPivotPro's FAQ Site

Read this post:  http://social.msdn.microsoft.com/Forums/en-US/sqlkjpowerpivotforexcel/thread/572168c3-7faf-41a6-88b4-790a9cd223d0

Also try this:

When my PowerPivot tab didn't show up in Excel, the following helped:
1. Open Excel and select File->Options->Addins->Manage COM addins.
2. Put a checkmark in the box for the PowerPivot for Excel addin and click OK.

If that doesn't work, then:
1. In Excel and select File->Options->Addins->Manage and select Disabled items in the dropdown.
2. Click Go and then select: Microsoft.AnalysisServices.Modeler.FieldList.Addin.Integration
3. Close and reopen Excel.

p.s. Thanks to Deva and Eric P for these tips.