Some folks are having problems with PowerPivot for Excel CTP3. In all cases we have seen, this is because folks have installed only Excel when installing Office 14 beta 2.
For Office 14 Beta 2 / PowerPivot CTP3, you must include "Office Shared Features" when installing Office. The reason is that VSTO runtime is installed as part of Office Shared Features and PowerPivot is a VSTO add-in.
If you installed only Excel and PowerPivot isn't loading, you need to
1. Uninstall PowerPivot & Office 14
2. Reinstall Office 14 and include "Office Shared Features"
3. Reinstall PowerPivot
That should fix things up.