Well, after much struggle and head scratching (one broken minidisc player and one dud optical cable) I finally got some test audio recorded from a minidisc via the Zoom U-44 into my mac. it wouldn't do it in Audacity, I have no idea why not. But, the Zoom came with Ableton Live Lite and that did, finally, record some audio.
It's been a bit of a struggle getting to this joyous moment, but I'm quite relieved it all worked. I haven't tried doing a DAT transfer yet. I'm going to assume it will work in principle and leave it to another day before I try it. I'm quite relieved, and now looking forward to transferring my minidiscs, finally. Phew