I have a D70 with a Nikkor 12-24 in an Ikelite housing. Maybe I can help ... I'll try.
Not sure where you are running into trouble so let me list basic steps and then maybe you can be more specific about where you are having a problem.
1. Mount the camera on back plate without lens, insert in case, and latch.
2. Slide/press sleeve onto lens with teeth nearest the camera end of the lens. I slide/press mine on until I can only see one row of the rubber lens grip showing.
3. Insert lens in camera and turn counter clockwise to lock in place. At this point the sleeve gear teeth should be engaged with the housing gear and you should be able to zoom the lens with the housing knob.
4. Put dome port in place on the housing and close the port latches. Test again to make sure that lens can still be zoomed with the zoom knob.
Given the above sequence, where are you having trouble?