ACR 8.6 requires at least Photoshop CS6 to run.
Found out that you can convert to DNG, and then it will work.
Sent from my iPad
Yes, you can use the free, stand-alone Adobe DNG Converter 8.6 to convert your raw NEF files to raw DNGs first, which you can then open in any version of ACR higher than 2.3.
But ACR 8.6 will never work with CS5.1, and you will never be able to open your raw NEF files directly in CS5.1. You will have to take the extra step of running the Adobe DNG Converter on every batch of raw files you take with your camera.
More importantly, you should be aware that you will be leaving a lot of image quality on the table if you work with any version of ACD 7.4—which also requires CS6 to run. The rendering in Adobe Camera Raw took a significant leap forward in ACR 7.4 and later versions.
It would be a real shame to work with an expensive Nikon camera and not obtain optimal raw conversions.