sudo port install nrg2isoin Terminal. Then, just execute
nrg2iso inputfile.nrg outputfile.isoin Terminal's command line. Two things may happen afterwards: console output may show the advance in the task of converting the original Nero .nrg file to the more standard .iso disk image format; alternatively, the .nrg file may already be a .iso file simply renamed to .iso and console output will warn us about that fact. In this case, we only have to change the .nrg extension to .iso.