You need the ACR 7.1 Beta from Labs.
I downloaded it,and PS CS5 still can't open my D800 nef files. I'm not sure what to do next.
Moved to the Adobe Camera RAW forum. Which Adobe application are you trying to open the RAW file in?
You need to download and install ACR 6.7, which will work with CS5 and with your D800. ACR 7.1 only works with Photoshop CS6.