You can either upload a mp3 file or link to a youtube link/video.
This is done by scrolling over add, then scrolling over music and video. Below it should show something that says music and video.
If you have a mp3 click music and then click the yellow button that says upload. Then once it uploads just click on the song. = )
If you have a youtube link click video and click enter url. Then just paste the link and it will add the youtube video to your screen.
Hope this helps!