Unfortunately there is currently no way to transfer tracks via NetMD from a Mac to a minidisc recorder. There are a couple Hi-MD recorders on the market that support high-speed uploading from MD to the Mac, but not the other way around. Sony's new MZ-RH1, due out this summer, is rumored to included better Mac support than previous Mac compatible Hi-MD units. What that means is anyone's guess, but it's certainly a possibility that downloading from Mac->MD will be supported.
For now you've got two options, either transfer in real-time via a digital/optical or analog connection, or boot into Windows and download things that way.