You just insert action replay and your game.
After that you turn it on... then click on the green *.
click on the cheats you want to activate but remember the cheat that says
must be on ... has to be on. If your device doesn´t has any codes for your game just hook
your actionreplay up to your computer and have a update.

Hope that was helpful.. :-D

You can´t use these cheats during multiplayer Luigi vs Mario.
The game will currupt and show an error message.
If anyone knows how to use the during multiplayer please post answer.

FOR GOODNESS SAKE ACTION REPLAY MIGHT PUT VIRUS ON UR NDS!

just put action reply in the ds and put the game in the slot at the top of action reply

Action replay isn't something you unlock in the game, it's a type of ds. (not by nintendo,
but by datel).
If you have a nintendo ds or nintendo ds lite, I'm pretty sure Action Replay WON'T WORK.

you buy it in the shop. put it in the gameboy bit (i think)
it works for ds and ds lite