You should be able to see a blue light behind the chair, which you need to shoot until it disappears. Walk forward, and stand in front of the gap between the two terminals, where the orange chair is on its side. Then turn right to run up another flight of stairs. Starting at the Pack-A-Punch machine, turn around and run up the stairs. NoahJ456 managed to complete this by searching for five blue lights, and shooting at them in order I've laid out below. We're now about to take a short detour from the main mission to check out the bizarre zombie 'coffin dance'. Once your Pack-A-Punch machine is up and running, you'll be transported back to the normal version of the world.
