Cleaning the registry refers to the removal of the CS 707-300 entry AFTER you uninstall it. It means that you have to activate the installer before you can install the aircraft again. The error that you are seeing can often happen if something went wrong during the last activation process that you did. Generally, you only have to active they installer if you have reinstalled Windows, on a new computer, or a new operating system. This is because the registry entry is not removed when you uninstall the Captain Sim add-on.

Mark