UPDATE - July 28, 2014
The NAM has returned to the Mac.
With the release of NAM 31 fast approaching, we'd like to update the Mac community at Simtropolis about the future of the NAM on MacOS. As many of you may know, the NAM has difficulty running natively on newer versions of OS X, beginning with Lion (10.7), due to the removal of Rosetta support. This omission prevents PowerPC-era apps like the Mac port of SimCity 4 from running on newer versions of OS X, without the installation of Aspyr's Universal Binary Patch, which has a number of quirks and instabilities, leading to inexplicable crash-to-desktop situations with seemingly unrelated combinations of NAM Plugins installed. We remain unable to solve this issue.
Additionally, as NAM 31 will mark a return to a "Monolithic" release paradigm, in which virtually all of the current separate-download plugins (e.g. RealHighway, Network Widening Mod, etc.) will be rolled into a single package with all of the "core" features, our releases will have much larger filesizes, such that the simple .zip folders in which we've traditionally released OS X versions of the NAM will become especially unwieldy in distribution.
To this effect, here is our new policy with respect to the NAM on OS X, with respect to the NAM 31 release, and until further notice:
The NAM Team is unable to provide technical support to users attempting to run the game natively on OS X 10.7 (Lion) or later, or otherwise using the Aspyr Universal Binary patch. As mentioned before, we have no solution, or even a hint of how to solve the related issues, and the Universal Binary patch introduces other quirks and instabilities into the game. Use the NAM on OS X 10.7 and later, and with the Universal Binary patch, at your own risk. The NAM Team will continue to offer support to those running OS X 10.6 or earlier without the Universal Binary, or using a dual-boot or emulation/kernel-layer system to run the Windows version of SimCity 4 on their Mac.
The NAM for MacOS .zip download will be discontinued for NAM 31, though it will still be possible to run the NAM natively on compatible versions of OS X. There will be one, single download of the new version--the Windows installer. The Windows installer can also be run under WINE, and it is possible to extract files from the .exe using Keka, a free extraction utility. The insides of the executable in Keka will look roughly like the existing manual install .zip, though upon extraction, you will want to rename the $[65] folder to "z___NAM" for the new version. Because the compression ratio on NSIS installers is even greater than .rar files, you'll have a quicker download than you would have with the old .zip.
Further details will be added as they become available.