Okay here's what you do (I downloaded code manager to make it easier for me to explain lol).
There are two ways to do this, but this way I think will be easier if you're only using 2-3 codes (which codes are you going to use, btw?).
Open up code manager, and change the Game ID to RSBE01 and change the name of the game to w/e you want (SSBB, for instance. This part doesn't matter IIRC)
Then near the bottom of the page (not the button on the very bottom, the button above and to the left of the one on the very bottom) click the "Add a code/comment/category" button. Change the name of the code to w/e you want, doesn't really matter much. Then cut the contents of the code and paste them into the "Code contents" area. Then click the "store modifications button".
Then once you've done that with all the codes you're using, click the "Export to GCT" button. Then put that saved file in the codes folder on the root of your SD card.