Trouble opening custom stories

[vendetta]

This might just be me being stupid, but I can't figure out how to open custom stories from the Amnesia launcher. I know I have them in the right place, but when I click "Custom Stories" it just goes to that folder, and won't let me open the game with them. I can't find anything documenting this problem and I'm starting to get a little hopeless. I'm using a MacBook Pro, Mac OSX Lion, and downloaded the game from the Frictional Games website, not from Steam- I don't have Steam on this computer. Please help!