You set launch options for a game in the properties dialog. You access a game's properties from the Steam games list.
The launch options for TF2 should be "-novid -window" by default. "-novid" causes it to skip the Valve logo video on startup. CrossOver has problems playing that video. "-window" tells TF2 to start in a window. There's a weird bug that we haven't identified such that if TF2 plays in full-screen mode, but at a resolution different from your normal desktop resolution, then the game screen is displayed off-center, making it hard to use. To avoid that, we have it start in windowed mode. Once in windowed mode, you can configure TF2 to use full-screen at your normal desktop resolution, exit TF2, remove "-window" from the launch options, and relaunch TF2. Then, it will start up in full-screen without the weird off-center bug.
You should not need to specify "-dxlevel 80" or anything like that. TF2 should automatically detect DirectX level 8.1, which is appropriate.
We have had good results with an iMac like yours, with ATI X1600 graphics.
What problems are you having launching TF2?