It's easy.  There are several ways to do this.  Here's one:

Let's assume that whatever music you want to use is in your "My Music" folder.

Open your slide show.

Click Media, then My Music, then a window will open listing your music (and music folders).  Select the music you want to use and drag it down to your slide show.  At that point you can move it, edit it and in general make it as good as you can. 

Alternately you can use File>Import>Browse> My Music.  Then select whatever songs or music you might want and then it will be available as slideshow music.  Good luck.