How to use my Nikon DSLR with Skype on a Mac

Nikon has "Camera Control Pro 2" ($$) and there is a program call Sofortbild (mac, free) which support "live view" and remote capture via usb or firewire. I have not used either.

Perhaps you merely replace the live camera cap software in step 2 of the page which you posted. The free software is easy to test...

(I use remote capture on a Canon DSLR from Breeze. They don't have Mac software for Nikon unfortunately, but AFAIK they use the Canon API which is what Canon EOS Util uses (the one in your link about doing this))

Nikon have Camera Control Pro software Which can actually control the camera remotely from a computer, but this software does not provide external interface for the camera. In the other words: your system will not know that there is camera, just some USB device. So, Skype will not find it.

Also there is some third-party software to control Nikon cameras that may include video interface: