okay that should be easy to fix... I use blender all the time, using it right now infact
what version of blender? (248 or 249 would be the right choice... 248 is python 2.5, 249 is python 2.6 if you care). Install/repair so as to have the settings in the blender directory for best nif exportation... sometimes can't find it otherwise.
you will need python (2.6 or 2.5 depending on your blender version) if not installed install
then install PYFFI version 2.0.0, if using python 2.6 you will need to manually install by extracting the exe (7zip will work) to a folder then command prompt there and type in "*pythonpath*/python.exe setup.py -install" replacing the *pythonpath* with the path to your python istallation, and no quotes ofcourse, if using python 2.5 just use the exe.
then (finally) install the niftools nifscripts for blender (version 249 or 248 depending on your blender version)
and now FINALLY you should be ready to export; just restart blender if it is open and it should be there... if not post again.
Pacific Morrowind