I believe this is a steam issue, as I just installed the steam client 
update from yesterday and the game no longer works on my system (e.g. 
the system where the mac version was developed). 
However, I managed to get it working again.. Here is what I did
- Delete Steam from my Applications folder. 
 
- Tell it "NO" when it asks to remove the local data (unless you really want to, you can :-D )
 
- Go
to the Library/Application Support/Steam/ folder and delete EVERYTHING 
except for the SteamApps folder (that is where your installed games are) 
- Re download steam from steampowered.com
 
- Reinstall it (drag to Applications folder)
 
- launch
steam and wait for it to update. it may take some time and may even 
claim you have no internet just after downloading updates.. just 
re-launch it again. 
- Once it finishes updating login, and validate your system (Steams authorization process as it thinks you are on a new system)
 
- Try
running amnesia.. It may take a bit as steam resyncs all your games (to
make sure they are all updated).. But it should run perfectly now.