You must have the correct cheat file and when you go to load your GBA ROM the 3rd tab in the M3 Game Manager software is for "cheats." Simply load the cheat file associated with your game and flash it over to your M3 Lite. The same goes for NDS ROMs, and the option can be found in the same tab when you goto flash an NDS game.
As for where to get cheats, try Google. I don't have a need to cheat in video games so I can't speak on that.
Goodluck,
-AW