Steam / Valve is basically a PC company that happens to have a Mac client.
I was happy last year when Steam came to the Mac, and appreciate that they give us Mac users access to a bunch of great games. But I've had off-an-on problems with the Steam client. Their problem reporting and resolution system is pretty bad. They're clearly a PC-based company with a PC mentality. They're not focused on the user experience, they're focused on getting their software to run and on cleaning up the inevitable mess (crashes, etc.). It's very non-Apple.
I've given up on Steam twice and left it and my Steam games alone for two or three months at a time. Then some great game will become available (I'm looking at you, L4D2 and Portal 2). The memories of the bad times will have faded and I'll jump in again, wide-eyed and innocent.
Then the (apparently inevitable) cycle of crashes, poor performance, searching the web for answers, modifying my setup, frustrating interactions with their customer service, joy at getting the game to run, rinse, repeat. I don't know if this is the standard for the PC side of Steam, but it's not what I'm used to.
As you may be able to tell, I'm now in the middle of that cycle and will probably swear off Steam in a few weeks after I complete Portal 2 or get tired of trying to get it to run.
In reply to your question, submit a ticket to Steam (a frustrating experience in itself), then check your e-mail in a few days for a reply. Reply to their misunderstandings of your situation. Repeat until done.