Mike,
You need a Nikon UR-E14 Adaptor Ring ( it is a short tube).
1. You unscrew a ring, that is around the lens, on the camera body
2. Screw this tube onto the camera body.
I bought the adaptor tube at Coxell. Hotline number 27290199. (approx HK$200)
You will then need a Swarovski Digital Camera Adaptor (DCA), as this is not usually in stock at photostores I sugget contacting Eric Hau, Swarovski Sales Manager, direct line 2969 6165. (under HK$1500)
The DCA comes with 2 tubes and a number of adaptor rings. The M52 adaptor ring will screw into the Nikon tube and one of the DCA tubes.
The other tube screws on the zoom eyepiece, remove the rubber eyepiece and now both sections of the DCA should fit.
Nikon 8400 camera, I suggest that you upgrade the firmware, because the firmware that ships with the camera will not allow the infra red control to do instaneous shooting.
Go to this website to download the appropriate firmware
http://www.nikon.com.sg/productitem.php?pid=81-96b71aa5d3
If you require further information in setting up the camera, just post your questions
Bob Thompson