A staff member has replied to your question:
In order to correct the crashing in Portal 2 please do the following:
1. Exit Steam.
2. Navigate to your Portal 2 bin folder (~Users\[username]\Library\Application Support\Steam\SteamApps\common\Portal 2\bin\)
Note: If you are using Lion please do the following to access your Steam folder:
- Click "Go" in the menu bar
- Click "Go to Folder..."
- Type "~/Library/Application Support/Steam"
- Click "Go"
This will take you directly to the folder.
3. Locate the libsteam.dylib file and drag it to your Trash Can.
4. Start up Steam, launch Portal 2, and try to begin a level.
The game should load without crashing.