That is because you other cameras are either shooting RAW+JPG or OS X has the RAW viewer for those built in.
Nikon NEFs are Proprietary and Nikon refuses to release the Real decoding system for it. It has been like that for years. Even Adobe doesn't have it and the only reason Lightroom, Bridge, ACR and PS can read those files is from Adobe Reverse Engineering the file format to some degree.
If you look at the discs that came with your camera, or on the Nikon website, there should be a viewer program which "MAY" also install a system for Preview to view those NEFs. But that is just speculation.
Not sure since you khave PSD CS6 your not using Bridge for all photo viewing. Right it's not Finder but hey you kgot to make do with what you have.
Other then that start shooting RAW (NEF) + JPGs with the D600. I know it can.