During the game, press esc. to get the main menu. type DOUGMATIC
several times until it drops you back into the game.
The cheats are now activated. Press esc. again

            AMAZON: For mission 2 "The Amazon"
            FRALES: For mission 3 "Frales Island"
            KILLEM - Kills Villians
            GIVEME - All Weapons
            LOADME - Unlimited ammo
            HEALME - Full Health
            PROTECTME - Invulnerablility
            MOVEME - Gives saved games of all levels
            AGENTJ - Become Agent J
            AGENTK - Become Agent K
            AGENTL - Become Agent L
            AGENTX - Become Agent X
            ARCTIC - Goes to Arctic Level
            HQ - Goes to HQ

            You can play MIB as anyone (even yourself)
            Just replace the files /art/3d/textures/head_f.bmp AND
            /art/3d/textures/head_b.bmp

            with any head bmp (even a pic of yourself). Then use the cheats to
            play as Agent X and Voila - you (or ANYONE) are in the game!