Yeah, I doodled with it for a while and saw the same thing. You need to add a slideshow even if it contains just one slide/picture, and then drag the music file over. Unfortunately, if you add multiple music files to the same slide show you can't skip songs during playback (no chapter access unless you add movies). Also, making multiple slideshows containing one song each lets you select individual songs but then you can't play them in succession!
I've read quite a bit about Toast, but never tried it. It does, apparently, support making music dvds.