First step:
install MacPorts (Direct access to the latest .dmg file for OSX Lion at the very moment of writing this post is
here). MacPorts is a set of software tools which allow distinct open source software from other POSIX-like systems such as Linux. These open source software pieces include command-line tools and X11 tools. It includes a software packaging system very easy to use.
So easy, that, we only need to install nrg2iso by executing the command
sudo port install nrg2iso
in Terminal.
Then, just execute
nrg2iso inputfile.nrg outputfile.iso
in 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.
