I bought Amnesia through steam, however when I tried to run it it would flicker the window "Preparing to launch Amnesia: The Dark Descent...". Checking my dock as it happens, I see the Amnesia game icon begin to appear then disappear. This repeats until I just CMD+Q Steam.
I have tried reinstalling Amnesia, that didn't fix it.
I verified the game files/cache thing, that didn't fix it.
I've downloaded all software updates except a silly printer one...
Can you start Steam, try to run Amnesia, then after you quit Steam open the Console that you find in Applications/Utilities/ and then under Console Messages (I think) look for lines that mention amnesia or frictionalgames and post those here, that might give a clue what it is doing when it flickers like that.
Do you have any other games on Steam? Do they work fine?
There messages seems really strange, starting/removing the game, can't find Steam and so on. Are you connected to the Internet when you try to run the game?
Maybe try to clean out Steam on your computer and re-download the Steam client from steampowered.com might fix it.
I also played Killingfloor, just not recently and I don't feel like waiting the tens of hours for it to patch back up so I could see if the same glitch is occuring.
Portal as well, but I have removed that game.
I am on the internet, but I tried it in offline mode once. All it does is make it flicker even faster.
I guess I should send a support ticket to steam themselves... =_=