iTunes Music Store files have DRM on them that prevents them from being read by "unauthorized" programs. There's no such thing as an "authorized" program for Linux, unfortunately.
However, since you have a powerbook running OSX, there's still hope. You can use jHymn (
http://hymn-project.org/ ) to strip the DRM from the file without any loss in quality. I used it on my laptop running Windows, and my iTMS songs play just fine on my Gentoo desktop. Give this a try.
I don't know of any Linux software that will unencrypt these files, but jHymn would work perfectly for what you need. Just make sure to make a backup copy of your music first...