You need (i) a sound recording program (MD -> .wav) and (ii) an MP3 encoder
(.wav -> mp3). I would recommend the combination of:
http://audacity.sourceforge.net
(for recording to .wav, via the line in on your soundcard)
and
http://lame.sourceforge.net
(AFAIK, the best mp3 encoding engine available)
If you are talking about MD->PC transfer using specific NetMD technology,
that's too bad, Sony has decided to make those transfers impossible because
of copyright issues. Hypocrisy galore, yes indeed, but it's like that.
Regards,
Wald