If you`re getting that type of error message, you need to make sure you`re entering in the codes exactly the way they appear on our website. Also, you must make sure you enter in the (M) code first, directly after you enter in the name of your game.
NOTE: This isn`t an issue with the very latest version of Action Replay GameCube, which offers all the codes preinstalled and does not offer facilities for inputting your own.