The .nif import/export scripts for Blender have not been updated in an eternity. The last version released was only compatible up to Blender version 2.49, and apparently Blender is already up to 2.6.
Hell, the .nif script site claims they're still working on compatibility for 2.5!

So what does everyone else use to edit their .nif files, since Blender doesn't seem to be getting support from them anymore?
Are there perhaps unofficial .nif scripts for Blender somewhere?

I still use 2.49. Maybe just me but I haven't seen earth-shattering improvements to need to upgrade.

I've heard others talk about using the updated versions, saving as blend files, re-opening in 2.49 to save as .nifs. I don't know but sounds like it would probably lose any unsupported features if you do that.

So the common thing is to just use the old version of Blender? That svcks pretty hard, but at least I know