When you get to the Cult At Dousm level, work your way though until you reach to the walker hanger. Once you are inside, open the big door with the control beside it. Then kill the man in the room inside the hanger. Turn on the lift in the hanger, then hop out of the room and go up the lift. Up there, you will see three walkers. Hop on one, then look down and press Y . Now you have the choice of the side cannon or the main weapon by cycling through the weapons. Press Y to exit.