i would suggest google is your friend, just search for mpc to mp3 converter!
i generally use the free tools i've had for years in the form of musedecdrop and lamedrop, musedrop converts the file to wav then lamedrop converts to mp3.
Try a freeware prog called Xrecode,it converts all manner of formats including mpc and is pretty easy to use i.e. I can do it! Just do a search for Xrecode and choose the freeware version.