Ohnar zoom reverser queries

I've just got an Ohnar zoom reverser but when I use t with a Nikon F65 OR D50 then I cannot see the image even when looking into a bright light.
Also, the area behind the 'white screen' has two slots one with spring clips, the other much wider - why?
I also not that with the D90 I lose quite a lot of the image, even though this item is supposed to be used with APS-C cameras!!
Any help welcome
Regards
Tony

For those as ignorant as me: that thing is a slide copier with an optic for image size reduction, so that a 24x36mm slide will be copied to APS size, added by some zoom range

Concerning the slits:
That thing resembles at is slide stage the Panagor slide duplicators.
The inner slit (with the big springs) is for installing the slide. The slide can be laterally shifted which makes sense for lateral cropping, in combination with zooming. The outer slit is intended for filters. Either foil filters or glass filters in 49mm mount. For the latter the slit has shallow, indented springs at top and bottom. Filters typically are neccessary for copying on film to adapt the light source to the copy film.
The diffusor is hinged to flip it down to make it easier to position a slide.

Concerning your other questions:
I guess the duplicator is not set for the focusing distance needed for the cameras you hung onto it.