As the Nikon D600 is a fairly new camera, support is not yet available. Whenever a new camera hits the market, both the camera manufacturer and Adobe must work to implement support for the new NEF files into Camera Raw.
In the meantime, Adobe has a free tool that can convert your NEF files to DNG, which Photoshop can read. More info: http://www.adobe.com/products/photoshop/extend.displayTab2.html?promoid=DTEHA
Howard is right, but there is one caveat: the DNG converter also needs to be updated for the new camera.
This issue solely exists because camera manufacturers insist in using a proprietary format that needs to be decoded.
Would they be using the free and open DNG format in camera, like Leica, Pentax, Hasselblad, Ricoh, your images could be opened on the day your puchased your camera in hundreds of programs, even in old versions such as Photoshop CS1. I've seen a preview of the D600, where it was stated that even Nikon's converters were not updated for the new camera at time of writing!
You will need to save the NEF files, create JPEGs alongside, and wait until Adobe Camera Raw gets updated (if you are using CS6) or the DNG converter (if using CS5 or earlier) to process the NEF files.
this did not work!
it does not recognize d600 nef files...
I apologize, merminy. I should have mentioned that the DNG converter also needs to be updated to support the Nikon D600. As with any new camera, support can take a while to properly implement into Camera Raw.
Yes, in the mean time, you might want to use Nikon's software, View NX to convert the NEF images to TIFF, and then process them in Camera Raw/Bridge.
The Adobe engineers need to spend some time with a D600 to add support in Camera Raw. (If Nikon has not sent prerelease versions of the camera before launch)
A Release Candidate of ACR 7.2 is currently available, I am not sure if Adobe will be able to add preliminary (unofficial) support for the D600 before it hits the final release status.
Hello! Good news for you: the wait is over:
The Nikon D600 just got preliminary support (it means that the color rendering might slightly change in future versions, when it will officially be supported) as of today:
I'm moving this question to the Adobe Camera Raw forum for specialized attention to your situation.