The typical file organization is to have the python/hoc and mod files together in a folder (directory). The gui way to launch is then to drag the folder onto the mknrndll icon and then drag the seed python/hoc file onto the nrngui icon.
If you want to do everything with a UNIX-like terminal window, then in the folder containing the python/hoc and mod files type
nrnivmodl python your_starting_file.py
This assumes you have /Applications/NEURON-X.X/nrn/x86_64/bin in your PATH (substitute the version number for the X.X).