Instructions
Position your character next to a wall in the zombie level using the left analog stick on the PS3 or Xbox 360 controller. Your right shoulder should be up against the wall while your gun is parallel to the wall.
Sprint along the wall while pushing the left analog stick forward. You can sprint in both the PS3 and Xbox 360 version of the game by clicking in the left analog stick.
Jump while sprinting along the wall by pressing the "X" button on the PS3 or "A" button on the Xbox 360. As you are in midair, quickly click in the right analog stick twice. This will cause your player to go prone and be caught on top of the wall. You can aim and fire from this position without being harmed.
Leave the wall simply by clicking the right analog stick and pushing forward on the left analog stick.