I Know others had the same issue here's info from a post from a couple months ago, I don't take any credit for it as im just copying it ***make sure you read the last part its very important***here you go, hope it works!(if your on psn I'd be happy to play zombies with you psn-RGCAG22)
"It has something to do with the Apocalypse DLC map.
There is a fix for this that I have done and it works. What you need to do is delete the game data utility for Black Ops II. This will delete the downloadable content packs and the campaign data, but it will not delete your trophies, multiplayer ranks or stats, or your zombies stats. The next step is to re-download the content packs with Apocalypse being the first one you download, then all of the other ones. Next, do a system file restore on the PlayStation as there might be some corrupted files. After doing all this, try Mob of the Dead and it should work.
P.S., do not click on the option "Restore PS3 System." That will wipe your hard drive clean."