Error when trying to enable the PowerPivot for Excel add-in for Office 2010 reads:
"Not loaded. The Managed Add-in Loader failed to initialize."
If the installation of the PowerPivot for Excel add-in succeeded, the add-in should be enabled in the Excel Add-ins options panel. If not, you might see the above error when clicking on the Manage -> "COM Add-ins" -> GO.
Unfortunately, the help provided in this social.msdn thread proved to be accurate.
Don't waste your time like I did uninstalling the PowerPivot for Excel Add-in, repairing office, reinstalling the Office Shared Features or Office Tools... and multiple orders of the above. Just uninstall the add-in, uninstall Office (altogether, it seems, is necessary), reinstall Office with the Office Shared Features all enabled, reinstall the Add-in.
It is silly, but that's the only way I got it to work.