I've been playing. It looks as if nif_import.py (and probably nif_export.py, as well) have been written by someone using Python v2.5. There are lots of constructs in the script that are not available in v2.3, which is what you get in OS X. Hence the errors.
I'm currently crawling through it, error by error, to find and fix them. If I ever get out the other end, I'll post the fixed versions. It will be surprising if the script(s) still operate properly after this exercise, though, as I am not a very competent Python coder.