Follow the guide
here
I've used this method to update my usercheat.dat many times on my own... only thing is code junkies seems to be behind with adding codes... I'll see if I can find you a better place to get the usercheat.dat or find the DDLI patch
Just found a really neat application for updating the Usrcheat.dat file called UsrCheatUp, which uses a nearby wi-fi connection to update your cheat data right on your ds (you don't even have to connect your ds to the computer to use it
, just run it like any other game rom!!!! )
I haven't tested it yet, so I will give it a shot and let you know if it works.
Here's the link if you would like to try yourself:
http://usrcheatup.googlepages.com/