In the new window, navigate to the folder where you've sotred the zip file with the nif plugin you've just downloaded and select it.Select the Import-Export category from the list on the left side of the screen, and then click on the button Install from file.A window like the one shown in the picture Blender User Preferences will pop up. In Blender, select the menu option File -> User preferences.

Take into account that this is not a official release but that it is still under development. Then you should install the Blender Nif Plugin 2.6.0a0.dev4.

Outfit Studio wrong possitions of the Cloth bones