I just got my SuperCard today and downloaded the DSONE EOS.. And I downloaded the latest cheat.dat file from cheats.gbatemp.net, and changed it to usrcheat.dat. I then placed the usrcheat file in the _dsone folder, turned on my DS, selected the game I wanted, and pressed Y twice. But the screen it brings up is completely blank, no cheats or anything there. And it's the same for every game I tried. The main game I tried it on is in the DAT file, Dragon Quest Monsters: Joker, so it should work...
Edit: scratch that, I got it working. I was going off the instructions on the DSOne EOS's website, that told me to download the CHEATS.DAT file, and change it to USRCHEAT.DAT.. but then I noticed there was already a usrcheat.dat file to download off cheats.gbatemp.net, so I downloaded that and tried it and it worked fine.