I may be oversimplifying this, but if you're trying to take something you've already recorded on your Minidisc and record it to your Mac's harddisk, then I would say you can do this in real time by simply using a standard cord with two small jacks from the headset output on the Minidisc, to the line in on your Mac. Then you can use a free app like Audacity to record, and can export the file (for instance to mp3).
In my experience, you can do this fine with a Powerbook, but if you have a Mac Mini, you'll need something like the Griffin iMic. The input from the Minidisc then plugs into the mic port of the iMic (the iMic is USB).
Or did I completely misunderstand the question?? 😕