ok.
go to myspace home. there is a menu list type thing near the top of the page, and one of the options is 'music'. click here.
on the right side of the screen, it should say 'search artists'. under this, it says 'keyword'. type the name of the band/singer you want in this box, and click search.
you'll get a list of profiles now, so click on the very first one. (this is usually the correct profile, but you can tell for sure by how many 'fans' they have)
now there should be a 'player' window on the right side of the screen. find the song you want in this box, and click 'add' (it says this under the song name.)
itll now say 'Do you really want to add this song to your profile?' click add song, and your done!
NOTE:
you can only add a song if the band/singer HAS A MYSPACE ACCOUNT. therefore, not all are available.
also, the band/singer you have chosen must have the song you want on their profile, therefore the one you want might not be there.