Verify Your System is Updated
Make sure your operating system and drivers are fully updated:
Click on the Apple in the upper left corner of the screen
Select Software Update...
Any updates available will be listed and able to be downloaded or you will be told your software is up to date
Verify Your Game Cache Files
This error is often caused by corruption in your extracted game files. See the Verifying GCF Cache Files topic for instructions to verify your game cache files.
Disable Background Applications
Please disable all of your background applications that could be causing interference with Steam. Internet Security software such as Little Snitch, Peer Guardian and Norton/Symantec Internet Security have been known to cause these issues. If you suspect that an Internet Security application is the root cause of the issue, you may want to consider fully uninstalling the Internet Security application