This page at Apple Support indicates you should be able to download Digital Camera RAW Compatibility Update 3.13 to support the D800 on Mac OS X v10.6.8 as long as you've got iPhoto installed (which you do):
If you haven't installed all of the Apple updates to your Mac, you should do so first. Always do operating system updates as they're pushed to you or as they become available. You need to do that in order to ensure that RAW compatibility updates install properly. If your operating system is already up-to-date, then just go ahead and download and install the RAW update 3.13.
As new cameras are released by various makers, the photo editing software developers and operating system developers scramble to create good quality RAW file converters as quickly as they can.