All you have to do is go to "insert" then click on "sound" all the way to the right, then just choose the music file to upload. You can also make it easier by just dragging and dropping your music file into the power point.
Click on the insert tab, and you should see something to insert sound. You then click the arrow next to that and click Sound From File. Make sure that your sound file is in the same folder with your presentation before you do this.