I can't open custom stories on my Mac! :(

[jens]

What do you mean with retail version? Where is the game from?
Check the moddb link that was given, you should use the terminal as it says like this:

open /Applications/Amnesia.app --args wn_config/main_init.cfg

You replace wn_config/main_init.cfg with different cfg files depending on what total conversion you want to run, you can see which on by opening the .bat file in a text editor.