Click on "Browse" and choose the song you want to upload from your computer. When you have finished choosing your song click on "Upload".
Depending on the size of your song and your connection speed this may take a few minutes.
You can upload up to 4 songs and they should be in MP3 format.
That's it, you should have a Standalone Player on your new MySpace music site now that plays your song.
If you're interested check out Weird Al's MySpace site