My experience using it on a Mac was very straightforward. First, I download and installed MacTeX. It is a huge download, more than 1GB. It may work if you get the smaller versions, but I never tested.

Then, I was as simple as downloading ABNTex and unpacking it on ~/Library/texmf/tex/latex . This directory structure may not exist, so you may need to create the folders after Library. 

Done. It worked fine for my monograph latex files.