This fix worked for me.

Close steam and navigate to - C:\Program Files (x86)\Steam and DELETE AppCache Folder, then restart Steam

Ps  if still not working after you have  deleted the appcache file start the game in single player mode  it will up date itself and your sorted.

THIS HAPPENED TO ME ON PS3 TOO INFINITY WARD R U SERIOUS FIX THIS PROBLEM BECAUSE EVERYONE HERE PAID $15!!!!!!!

I dont know about anyone else but deleting a file bothers me.  I know it probably works but why is there not a patch for this.  I bought the DLC 2 and all and a buddy of mine gets is DLC2 is not installed, you have to get the map pack.   I already have it.  I have never had an install or download problem.  Can a person cut that file and place it in another folder somewhere just in case it does not work? Of is that a files that should not be there in the first place?  Also when I check donwloadable file list DLC 1 shows in single player but not DLC 2 and neither shows up in multiplayer downloadable file list.  Will deleting this file after closing steam resolve both issues?  I get the Sanctuary is missing or damaged when I go into the new DLC2 play modes, but in HC deathmatch It says DLC 2 is not installed, yet all the componnents seem to be there.  Why is there not a patch?