You need to right click on the object (or on the node in the list) to bring up a context menu. I wasn't accurate on the menu designations tho.
While i was at making the screenshots also moved up the Airship. Didn't test the height in the game, might still be too low.
Also the prop animations might glitch. But i hope they won't.
Note, its more than one object - the "Effect" Dummies are particulary easy to miss. You need to make NifScope "DrawNodes" and "ShowHidden" to spot everything when doing this kind of changes.
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On _FX files.
Each Unit Art-define has 2 tags - Nif and ShaderNif - the ShaderNif is the FX one. By default Shadernif will be displayed.
The Fx nif should look better (Lightning, Gloss, ...) and perform faster on adapters capable of showing it (All but the crapiest old on-board chips should be), so it's preferable and will be used, unless the game find out that your card can not draw it.
Custom units usually only come with one Nif, which you then use for both Shader and NonShader.
Unfortunately its the the No_FX one most of the time.
Not exactly sure abouth these freeze thingies - the numbers corresond to some key anim sequences - Idle, Run - so i think they are supposed to be used for Frozen Animations on even weaker computers.