To back up what Steve has said, MacOpener (and other programs such as MacDrive) only allow your PC to talk to your iPod as if it were a removable hard drive - not a music player. If you send music to it, you are in effect copying the mp3's to the root of the iPod as data, not as music that can be played. The iPod has a "music folder" that requires a program like EphPod or XPlay to talk to it. I've never tried EphPod, but I hear that you have to have MacOpener to make it work, so it seems like a viable solution for you.
I use XPlay to put music on my iPod. It's fan-freakin-tastic. I use MJ8 to tag and organize my music, and to create playlists. Then I export those playlists to .m3u's (a process that could use a little upgrade -- things like auto-synching of mj playlists to external m3u's would be sweet), and then I use XPlay to send those playlists to my iPod. Works like a charm.
It's true that Mediafour is working their butts off trying to get XPlay ready for their release date. They're currently in beta3 r1, and the beta will expire on 23 June. Just by looking through some of the posts in their beta forum, you can totally get the sense of urgency coming from them. It's a great program so far. You can find their forums here:
http://www.mediafour.com/forums Rock on, iPodders!