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.