Please help! ATDD won't run on my mac

[heleneking]

Hello,

ATDD won't start on my mac - I see the Frictional Games logo, and then I can hear the sound effects, but the screen is completely black. At this point I'm unable to do anything to exit the screen or access my desktop, and have to do a manual shutdown.
Through reading all the threads I know others have had this problem, and I've tried various troubleshooting suggestions but never seem to get very far due to being generally ignorant about (and a bit scared of) computers (meaning I can't convert instructions for windows to those for macs, etc...I can't work out how to update my graphics card, for example).

I'm running OS Mavericks 10.9.1, my graphics card is NVIDIA GeForce 320M 256 MB. I bought ATDD through Steam.

I've already asked Steam for a refund out of sheer frustration, but they won't give me one unless I ask for help here and show them the evidence. I'd much rather get the game working, obviously...so, if anyone could help me with an idiot's guide to solving this I'd be extremely grateful....