Sorry, but Niftools nifplugin does not support Max 2012.
Development has been halted for some time on that nif plugin, in regards to Max, because Niftools team still lack a Max developer for it.
I don't know when development might continue...
I had heard something a while back about someone who might help them with furthering the nif plugin support for Max, but I've heard nothing since then.
Your best bet is to try Blender 3D modeling program, for now.
Koniption