Trust me, it couldn't be easier. I assume you know there are various music file formats? Studio 8 likes mp3s and wav files, I'm not sure if it accepts other types.

Put them in one folder, just as if they're video clips. With your Studio project open, navigate to the folder (the 'album' bit at the top left) so that the music files start to be listed.

Next, do one of two things: either drag your selected music file down to one of the sound tracks (the bottom two are for sound files). This'll drop the whole song, so you'll need to shorten it (edit it), same as with video clips, to fit your project.

Or edit the music file in the sound window: double-click the music file (the icon in the album part) and I think it opens the sound edit window (not so sure about this, you'll have to try it.) The sliders below will alter the in- and out- points of the song. When happy, drag the edited clip down to the music track.

Sorry to be so long winded, it's far simpler than it looks. Give it a go, and post back.