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.
Game Shark Codes
Master Code (Must Be On) ECB8850C 143AFE1C
Infinite Batarangs 4C2064CA 1456E79C
Infinite Batcuffs 4C2064D6 1456E79C
Infinite Stunners 4C2064CE 1456E79C
Infinite Remote Charges 4C2064D2 1456E79C
Infinite Nets 4C2064D4 1456E79C
Infinite Elec. Batarangs 4C2064D0 1456E79C
Infinite Flash Bombs 4C2064D8 1456E79C
Unlock All Levels 3C13F3E8 1456E7B4