First of all you need the music. It has to be a 44.1KHz, stereo MP3. Some will say it has to be sampled at 112kbps but it doesn't.
Place this file in music/<yourmapname> folder.
Open the map in rediant. With nothing selected open up the Entity window (N) and scroll down to Worldspan and select it. Now enter he key "music" and the value "<yourmapname>/<yoursundfile>". (Or "music/<yourmapname>/<yoursundfile>"? Can't check now.)