I think it a user thing ! If you want to add just one song, you need to use the "ADD1" command. If you want to add a selection, you need to be in the level below the selection you want to add, and then use "SELECT+".
So, assuming you have your music structured like
This threw me for a while until I played with it for a bit, but if you want to add an album, you highlight a track, and use "SELECT+". This will add all the tracks at this level to the playlist.
Hope this helps.