Enter these codes at the "Code Entry" menu. You can find it under "Extras".
Effect - Password
1-Hit Dishonor - 54641
Character Concept Gallery - 97831
Double Damage - 34743
Neverending Stealth - 53467
Unlimited Chi - 24463
Unlimited Copper - 23637
Unlimited Health - 94677
Unlock all Treasure Maps - 37437
Defeating the fire machine
This is the machine you must fight in order to find Katara. Dodge the fire bending. Try running left to right, then right to left. Continue this until you see a flashing light near it, then attack it. Hit it with Aang. You may use a Chi attack but just a plain attack should work just as well, however it takes time. You will have Saka (Katara's brother) fighting with you to do double damage. Once it has around a quarter of its HP gone, the machine will start throwing fire around. Keep a safe distance from this attack. It causes no real threat to your or your partner as long as you stay as far away as possible.
Defeating the Maker's last machine
You can wait for it to stop attacking, or you can attack it, which will barely affect it. Once it is done attacking, you must attack the sides which turn red.
Easy experience
Whenever you are in battle you can go back later to get more experience.