Suggest downloading FCS Remover. Check Compressor in the dialog and run it – deleting both the application and related plist files. Then download/install a new copy of Compressor from the app store. If you have custom settings you want to retain, before you uninstall, go inside the Home library (hold the option key when you click Go in the Finder. In Application Support, open the Compressor folder and move the Settings folder to your Desktop. When your newly installed app is working properly, you can replace the new Settings folder with the old one.
Good luck.
Russ