If you're using the original loader:

http://www.ndsthemes.com/themes/title/m3real
http://www.ndsthemes.com/themes/title/M3

I can't remember off the top of my head exactly how the structure works, but if you go into the system folder it should be fairly obvious.

If you're using Sakura (if you haven't tried it yet, I'd recommend it), all the information you could possibly ever need is at:

http://gbatemp.net/index.php?showtopic=89197