When you first get to the main menu, you want to keep pressing R2 and L2 until you break out. Then, you want to turn around and go to the back of the room and you should find a computer on a stand. When you go on it you can type in cheat codes. For all of the Intel, you want to type in 3arc Intel. For all the zombie maps,(except ascension) you type in 3arc Unlock. Type in these cheat codes and have fun!

For a list of codes, see: http://www.gamefaqs.com/xbox360/960189-call-of-duty-black-ops/cheats