When I am trying to export .nif, I get this error:

Or sometimes it's "need more than 1 value to unpack". How can I fix it? :S

What game nifs you are trying to do?
As I know Nifscripts currently work only on Blender 2.49b (Fallout 3 recommended).

As front210 mentioned you need to use 2.49b for the import/export scripts.

However, its your choice if you want to work within 2.49 exclusively. I know a few (myself included) who import the file in 2.49, save it, and then work the blend in 2.7x. When done, save it in "legacy format" to open it back in 2.49 and export.

You get a warning of some animation data getting lost in the 2.7x > 2.4x process, but I've never had a problem. Then, again, I've never exported animations.