If you go into "project -> project settings -> general" make sure the "GM" button is pushed down.
Furthermore, you have to turn you desired levels into game master levels. go into "level -> edit level data..." Make sure to turn "Is Game Master Level" to true.
That is at least all that I did and that seemed to work.
Well, that and when you create a new campaign in game, you need to tick on which mods you will be using for that campaign, or none of the levels will show.