First put on the Low Gravity cheat (on this site) and play Zombies.

Next kill zombies until they drop grenades. Now get on the top of the fountain and run and then jump and push the left stick up.

When you are there go to the little corner and throw a greanade down and jump.

Your guy will get stuck so throw another greanade and you will need to go to the edge and jump, but do not open your parachute.

The game will then start to load and you will be on the streets.

You can not kill people but it is cool. To exit just press down on the pad.