"You must have installation media for the Enterprise, Evaluation, Developer, or Data Center editions of SQL Server 2008 R2. "
That means that PowerPivot on top of SQL Server 2008 R2 Standard Edition is NOT possible.
Install PowerPivot in a new farm
Install PowerPivot on an existing farm
- PowerPivot for SharePoint adds SQL Server 2008 R2 Enterprise files to your SharePoint application Server