Here's a rundown of how to use cheats.
Everyone connecting to the game needs to have the same cheats as Zaq stated.
"Set" cheats you need to pause the game or enable them at the CMOS screen that has an error. The game is paused at this screen. If you need to enable the cheat after you pick your character then you MUST use pause (P key).
"On/Off" cheats can technically be enabled anywhere and then rebooted. This has proven to cause errors sometimes though. Safest place is the CMOS screen stated above. Pausing is not suggested because it causes desynch sometimes.
Rebooting (F3 key) after you enable the Cheat is required sometimes, EG for unlockables. If you're having to pause the game to enable the cheat you don't reboot. EG Enabling the cheat after you choose a character like playing as Noob Saibot.
The cheat that everyone uses for unlockable charactgers is by MOE30W, it's an "on/off" cheat and should say Unlock *character name* By MOE30W. I have never had a problem with this cheat desynching.
and here they are (just place these in a text file and name it cheat.dat and keep it in your mame directory)
Code:
umk3:0:02810FC:ED:110:Unlock Mileena By MOE30W:Press F3 to reset after you turn it on
umk3:0:02810FE:B0:610:Unlock Mileena (2/2)
umk3:0:0281104:ED:110:Unlock Ermac By MOE30W:Press F3 to reset after you turn it on
umk3:0:0281106:B0:610:Unlock Ermac (2/2)
umk3:0:028110C:ED:110:Unlock Classic Sub-Zero By MOE30W:Press F3 to reset after you turn it on
umk3:0:028110E:B0:610:Unlock Classic Sub-Zero