I've solved the problem of the MIDI file which consists only of rests. (Thanks are due to Joe Offer.)I opened the nwc file and copied the music using control +C. Then I opened a new file, pasted the saved notes using control+v, then Saved As a .mid file. It worked.
Apparently the nwc notes were music-flavored, and it wanted them to be electronic-flavored - or something like that.
Nigel: who's Mike Batt? Guest: I will check out alt.music.folk.