Did a project recently for work in imovie 08 not sure if it is exactly the same but......
import movie as new or existing event then start new movie project, pull movie from event into new project. if there is existing sound with movie go to the beginning of clip and and hover cursor on the first frame it will show a speaker click on it and turn down the sound all the way. now on the right there are four buttons a music note will be one of them, click on it. this will open up screen to show music on computer, it will show Garageband Itunes and any other music programs on computer. open Garageband or Itunes and a drop down screen on bottom will open from there click on a song and simply drag onto the bottom of, meaning lower portion of the movie clip. the music clip will show up as green strip, from there click on the green strip and it should highlight music then grab front and pull to front of movie clip and then go to end and match up.if music clip is longer it will cut off end. if you need. you can go to garageband first and and modify music clip to exact length then save and go back to imovie and open music note and follow above. hope this helps..