You need the lame and mpcdec commands. On Debian, mpcdec is in the musepack-tools package:
sudo apt-get install lame musepack-tools
Then to convert all mpc files in the current directory to matchingly named mp3 files:
for x in *.mpc; do mpcdec "${x}" - | lame -r - "${x%.mpc}.mp3"; done
Oleander Donovan's IT blog 
Sorry to hear that. Please let me know in case you manage to figure out what the problem was.
It doesn’t work for me. Generates little MP3 files (around 1,3 KB most of them). I’ll try to convert to WAV instead and then to convert those WAV to MP3 :-/ Output of the command gave little information (“An error occured while decoding”).