When in the character screen, click the "Secret Code" button 
then enter one of the cheats:

Code                 Result
---------------------------
seventh chakra      - unlocks aang.
painted lady        - unlocks katara.
metal bender        - unlocks toph.
breath of fire      - unlocks zuko.
western air temple  - receive 500 gold.
boomerang           - receive 500 gold.
mai                 - receive 500 gold.
metal bending       - unlocks Toph.
painted lady        - unlocks Katara.
Gondola             - to unlock 500 gold.
Dragon              - to unlock 500 gold.
aang                - seventh air temple
lullaby             - 10000000000 money