Here are some codes I found on the net: 111-111 - Free Weapon - One weapon drops onto the center at the beginning of the match.
100-100 - Throwing Disabled - No throws or bone-breakers.
444-444 - Armed and Dangerous - Start off with your weapons drawn.
666-666 - Silent Kombat - No background music.
050-050 - Explosive Kombat - Last hit causes loser to explode.
222-222 - Random Weapons - Start off with another weapon at random, rather than default.
123-123 - No Power ?? - Start off with just a bit of energy above DANGER level.
555-555 - Many Weapons - Weapons drop all over the stage.
333-333 - Randper Kombat - No longer works in the home versions.
060-060 - No Rain - No rain on the Wind World stage.
002-002 - Weapon Kombat - Both Players never drop their weapons.
???-??? - 4-Player Randper Illegal - ???
012-012 - Noob Saibot Mode - Used for the process of playing as Noob Saibot on the Playstation.
020-020 - Red Rain - Causes it to rain blood at the Wind World stage.
010-010 - Maximum Damage Disabled - No Maximum Damage limit for Combos.
110-110 - Throwing and Maximum Damage Disabled - No throws and Maximum Damage Combo limit.
011-011 - Kombat Zone: Goro's Lair - Play at Goro's Lair stage.
022-022 - Kombat Zone: The Well - Play at The Well stage.
033-033 - Kombat Zone: Elder Gods - Play at the Elder Gods stage.
044-044 - Kombat Zone: The Tomb - Play at The Tomb stage.
055-055 - Kombat Zone: Wind World - Play at Wind World stage.
066-066 - Kombat Zone: Reptile's Lair - Play at Reptile's Lair stage.
101-101 - Kombat Zone: Shaolin Temple - Play at Shaolin Temple stage.
202-202 - Kombat Zone: Living Forest - Play at Living Forest stage.
303-303 - Kombat Zone: The Prison - Play at The Prison stage.
001-001 - Unlimited Run - Get an unlimited run meter.
313-313 - Kombat Zone: Ice Pit - Play at Ice Pit stage.
321-321 - Big Head Mode (No actual on-screen text) - The characters have over-sized heads

But I don't know how to use them. Can someone tell me?