First thing you need to do is exit steam... Then you need to make sure that Steam isn't running at all in your processes.
Then go to C/Program Files/ (x86)/ Steam and delete the AppCache (or drag the folder to the desktop.. That's what I did) x outta that then open steam. Go to library-Right Click Call of Duty: Modern Warfare 3- Multiplayer and click on properties- then click on the Local Files Tab and Verify Integrity of Game Cache after it's done verifying then open the game and search DLC 2 games.
This is all my husband and I had to do and we were able to get into the new DLC maps without getting any errors.
Best of wishes && good luck