Hi, I have a D800 and just now verified by booting into Snow Leopard, and Aperture 3.2.4 that its NEF files are compatibile. Furthermore I checked to see the latest Digital Camera RAW it had been updated with -- it was 3.14.
In addition, my memory is that I once did a special test having found 3.11 to have been withdrawn, and that updating to 3.14 without having made the 3.11, supported the D800.
So, Leonie's question about the latest update you have made is key to resolving your issue.
I currently am using both Lion on one Mac, and Mt Lion on another, and thuse much later DCR updates, but the D800 was supported quite early, and should work on your system.
Ernie