First thing to check, do you have the latest version of Circle installed? It's always available here: http://www.futureaudioworkshop.com/circle/downloads/
If that doesn't work...
- Copy your custom presets on to the desktop. The presets can be found in (Mac) "Macintosh HD/Library/Application Support/FAW/Circle-2/Presets" or (Windows) "C:\Program Files\FAW\Circle2\Presets".
- Delete the Circle program folder: (Mac) "Macintosh HD/Library/Application Support/FAW" (Windows) "C:\Program Files\FAW\Circle2".
- Reinstall the latest version of Circle, it's always available at www.futureaudioworkshop.com.
- Reload Circle and you should be able to activate successfully!
- Copy the custom presets that you place on the desktop back into the Circle program folder.