There are a few things you'll need. The first is a Minidisc player with optical out. You will soon find, if you have not already, that nearly all portable Minidisc players/recorders do not have optical out. The only exceptions, afaik, are the $1200 Tascam unit designed for reporters (in lieu of a DAT, presumably) and the original Sony portables from way back when. Before you jump in to correct me, please note the distinction between optical IN, which all MD players seem to possess, and OUT, which is only on the aforementioned portables and home decks. (But if you do find a reasonably priced recorder with optical out, please do post and tell me and everyone else -- I would snap up such a beast without a second thought.)
The other thing you will need is a way to get the optical audio back into the computer. I use a Roland/Edirol UA-30 for this task (and also drive my speakers off of it/use its analog inputs). It's about $160, although simpler units that only do optical in/out may well be much cheaper.
Good luck.