Open the Inspector and click on the "Document Inspector" icon in the icon bar at the top of the Inspector (first icon - see image on the left).
Make sure you have selected the "Audio" tab in the "Document Inspector" section. This will open a new window to your iTunes Library. Here you can select your song. You can preview it by clicking the "Play" button at the bottom of the window. You'll note your songs are listed with their play times so you can better judge how long a piece of music to select.
All you need to do is click and drag the piece of music you want into the Soundtrack window. You can set the volume with the slider.
If, however, you want to change the music on each slide, that's a different, and more complex issue and a topic for a future post.