This is the niftools plugin for Gmax.
Gmax is a free version of 3dsmax4 which is hosted at Turbosquid.
With this plugin you can
-create and export static or rigged models and animation for morrowind, excluding particles or morphs
-import nif files. skinned meshes usually do not import correctly.
-other nif versions are also somewhat supported too such as oblivion, civ4 and so on but who cares
These files were bundled with the 3dsmax niftools plugin releases on sourceforge http://niftools.sourceforge.net
however are no longer hosted there, you can find them here https://github.com/niftools/max_nif_plugin/releases
The files i include are just the gmax files from my own installation. I'm not sure which release version it took me some trial and error, and even mis matched fles from different versions until it just worked.
Both gmax and this plugin are dead projects with virtually no support/bug fixes/updates/etc. you are on your own
I personally recommend to use blender with G7's nif plugin over this, but for some reason you can't or won't this is an alternative if you are determined enough to make models with it.
To get Gmax you will need an email, make an account at turbosquid, download and register Gmax, you will receive a registration code in your email.
Its a little annoying i know but its free.