The easiest way that I have found is to put the mp3 files in specific folders on your SD card.
Alarms go in: sdcard\media\audio\alarms
Notifications go in: sdcard\media\audio\notifications
Ringtones go in: sdcard\media\audio\ringtones
As soon as you put MP3, MID, or WAV files in those folders you can go and change a ringtone, notification sound, and alarm sound. They show up in the regular tone listing.