D600 raw files only open in CS6/ACR7.3. You need to upgrade your Photoshop. http://www.adobe.com/photoshop/
An alternative is to use the free DNG Converter to convert your NEFs to DNGs and those will open in older ACR versions. Another alternative is to get Lightroom 4.3.
I have owned 5.1 Photoshop for less than a YEAR! When I bought it they said that it would be at LEAST a year till 6 came out and less than a month later TA DA 6 came out! GURRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR
Now they want me to upgrade again.
If it really was only a month before CS6, then you may have been or may still be eligible for a free upgrade, but perhaps that was only for a limited time.
I bought my 'upgrade from Cs3 to CS5 on Jan 31 2012...I had CS orig for years before that. Order # AD004671634.
Do you know when they actually released CS6 I KNOW it was VERY shortly after I bought CS5 and I was VERY frustrated...I did not know that I could get the free upgrade! UGHHHHHHHHHHHHH