Now I have the problem that always comes the following error message: Error when creating patched vehicles_airport, restoring from backup now!
That's not really an error. It either shows that your default vehicles_airport.bgl file is not original anymore, so the installer won't patch it, or it just mean it's missing, because it was removed after you initially installed GSX.
Since the GSX installer obviously made a backup before trying to patch it, that message is simply saying that is restoring from the backup.
If your file is not default, there's nothing else you can do, the file will not be patched (if you used 3rd party ground vehicles replacement sets)
If, instead, the file was missing, now that GSX has restored it from the backup, you might try running the GSX installer again to let the installer another chance to patch it.
The only reason to patch that file, is to remove the default pushback truck, so you won't see both at a parking. It's not usually a problem, since starting positions are usually different in GSX, it's just a cosmetic issue.
And I can not activate because to me the Addon Manager does not appear in the task list above also.
That's an entirely different issue that doesn't have anything to do with the above, and doesn't have anything to do with our products installation either, it's a result of an incomplete P3D installation, as explained here:
http://www.fsdreamteam.com/forum/index.php/topic,10084.msg79077.html#msg79077
