#1. "RE: Problem choosing AF-Area Mode in my D800" In response to In response to 0
Winter Springs, US
That is because you need to first use the rear knob to set the camera to AF-C. In continuous focus mode, you can then use the front know to choose 9,21,51, or 3D. In AF-S you only have the choice of AUTO or "S".
I hope this helps. The whole focusing thing was so confusing to me at first that I finally had to take a pen and paper and diagram it all out.