Go to the Gasworks level. You will reach a room with two levels, each floor with two goons on them. First, take out the goons. Then, look around on the lower floor until you see a sewer pipe with boxes in the sewage. Carefully glide down and land on the first box. Jump over the next few boxes and pick up the envelope with an "E" on it. Finish the game and the cheat will be unlocked.