Hmm, i have the same problem, except i tried out what rayder suggested and haven't had any luck.
I save my cheat.dat(which has the same content as usrcheat.dat) as chat.db replacing the old one in the flash rom(btw, it does let me save as cheat.db, but it says "*.dat" in the box), but when i go to the game's cheat page, it says that there are no cheats for the said game. I checked and the game ID is what it should be(AVPC), and i only added one code(Max luck, which i converted from the us version to 01211F45 0000E4A9).
Btw, i als tried to save the codes as a text file, tried to activate them but the one i wanted doesn't work at all, and the moon jump works weird...