^^Polaris has it. The reason why it's really hard is because they think the Phatnoise Music Manager is like iTunes or something but it is a really crappy iteration at that. I mean "like iTunes" is that they think that all your music exists in one place and you just "sync" the stuff you want on your DMS and put it in your car. You "sync" by playlist, genre, artist etc... So in the super ideal world of the Phat you would just create new playlists, select the songs and it would create new and lose the old... reality is that we freak out over losing files, we don't want to be that confined, and we don't have all our tunes in one place. So yeah, click on your "my computer" icon if you're a Windows user and select the larger partitioned section of the DMS hard drive (I think it's PHDTA) and explore for your songs.
Of important note, I would only COPY and paste and not CUT and paste because the phatnoise looks for the files along with the audio files it creates to announce the artist, genre etc..., but the main reason why you don't want to add or remove files without the Music Manager is that when you load the DMS back on the cradle and Music Manager starts it looks for the files you removed and will throw your computer into a vicious cycle of "Not Responding." This sucks really bad.