You have to convert your song to .WAV (Any music converter should be able to do this). Now find the slide transitions tab/window and there is a part where you can insert sound with a transition. Click on the drop down box (sound) and you will have stuff like 'bomb' 'applause' ect. One of them says other sound, click that, find your song and click ok. Only put this on the slide you want the music to start playing on. If you want a new song after it, download Audacity and put the next song after the first song, convert it, and put that in instead of just the one audio file. Good luck :P