I just tried it, myself, with a brand new adventure (made a new folder, set the level directory in the editor, and hit save) and saw the same sort of behavior. I'm betting that's a bug that we never encountered because we only made the one adventure. That adventure's .dat file contains a field for the level directory: LevelSetFolder:Levels For Tidalis Main Story. When I created my new adventure, there was no such field, so I think that option in the editor simply doesn't save whatsoever. I tried manually setting that field in a text editor and that seemed to work, so give that a try and see if it works for you.