Trying to use PowerPivot, just found undocumented PowerPivot installation requirement: Excel .NET Programmability Support.
If PowerPivot silently fails to load, and in Event Viewer you've got an event like below, run Office Setup / Add or Remove Features, then check .NET Programmability Support under Microsoft Excel.
Here is the event you might find in event viewer:
Customization URI: file:///C:/Program Files (x86)/Microsoft Analysis Services/AS Excel Client/10/Microsoft.AnalysisServices.Modeler.FieldList.vsto
Exception: Customization could not be loaded because the application domain could not be created.
************** Exception Text **************
Microsoft.VisualStudio.Tools.Applications.Runtime.CannotCreateCustomizationDomainException: Customization could not be loaded because the application domain could not be created. ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.IO.FileNotFoundException: Could not load file or assembly 'Microsoft.Office.Interop.Excel, Version=126.96.36.199, Culture=neutral, PublicKeyToken=71e9bce111e9429c' or one of its dependencies. The system cannot find the file specified.
File name: 'Microsoft.Office.Interop.Excel, Version=188.8.131.52, Culture=neutral, PublicKeyToken=71e9bce111e9429c'