Q: Mac OS 10.9.4 cannot open the NEF/RAW files from the new Nikon D810 Camera

Here is the list of cameras supported ny Mavericks OS X Mavericks: Supported digital camera RAW formats

If your camera is not on the list then it is not currently supported.

regards

If you will take the time to send Apple feedback, they will add RAW support - when I got my Canon, it wasn't supported either. I filed feedback and, two months, later, there was an update which included my camera model.

http://www.apple.com/feedback/macosx.html

Good luck - with so many new cameras being introduced all the time, I guess a lag is to be expected....