If you get an error message in Outlook that reads "Could not install the custom actions.” here is the fix:
1. Close Microsoft Outlook.
2. Make sure hidden files and folders are shown. This can be done in "Folder Options" within the "Control Panel" on the "View" tab.
3. Navigate to the following location:
C:\Documents and Settings\[User Name]\Local Settings\Application Data\Microsoft\Forms\
4. In there, there is a file called "FRMCACHE.dat". Delete this file.
5. Relaunch Microsoft Outlook.
More can be read about the fix at: