Download it
here. It's for the R4, but anything that uses usrcheat.dat or cheat.dat should work for it. Anyway, it works like this:
1) Open the cheat database (usrcheat.dat or cheat.dat)
2) Click the binoculars and find the ROM you want to insert a cheat for
3) Copy the Game ID that comes up after you find the ROM
4) Click the DS with green check mark symbol
5) Enter the Game ID you copied before and the game name, then press OK or whatever it says
6) Click the button between the folder with green check mark and scissors
7) Input a name for the cheat (ex: Infinite Ammo, 100% Catch Rate, whatever), a note about the cheat (what it does), and the Action Replay Code. Click Update Cheat Database or whatever when you're finished entering a cheat
8) Repeat steps 6/7 for adding more codes
9) When you're done, save the cheat database.
For action replay codes, just use Google. If this doesn't work, post whatever flashcart you're using.
Have fun!