>I recently purchased a Nikon D4 camera. I have Capture NX2 >2.2.6 on my computer.
Me too, and I anticipated the problem which you are having. I have an Early-2009 unibody MacBook Pro 17" with anti-Glare Screen, and 8 GB of RAM. This is a 2.93 GHz Intel Core 2 Duo based Mac currently running Mac OS X 10.6.4. I upgraded the RAM from 4 to 8 GB when it was just over a year old. (8 GB of RAM was a $1200 option from Apple when I purchased the computer, but the parts to make the upgrade are now available from OWC for $49.79. OWC provides free instructional videos too.)
I copied my early D4 NEF and JPG files to my Mac from my Sony QD-H32 card using the Sony XQD USB Card Reader and my Mac's Finder (similar to Windows Explorer on a computer running MS Windows).
I can open the NEF files using Capture NX2 ver. 2.2.6. I can edit the NEFs but cannot then save them as NEFs. I can, however, save JPGs made from the edited NEFs.
The Nikon Site isn't very explicit about which versions of Mac OS X are required to support particular versions of Capture NX2. I'm behind on my MAC SW Maintenance but I recall that I need to update to OS X 10.6.8 first in order to update Capture NX2 beyond 2.2.6.
I think that you need to update your Mac's OS X to at least 10.6.8. If it is a Powerbook G4 I think that it is at the end of its rope. I think that you can't update it beyond 10.5.8 since Apple dropped support for the Power PC Chips as of Snow Leopard, 10.6.0.
Make sure that you have at least one fresh bootable clone of your Mac's Hard drive before you make the update to Snow Leopard. I'm very happy using Carbon Copy Cloner to make and to update clones. I recommend that you use it with an external Seagate drive via the Firewire interface. Drives from other manufactures work too, but there is known to be incompatibility which prevents booting from some models of Western Digital hard drives.
Once your Mac OSX is at 10.6.8 v.1.1 you can update your Capture NX2 beyond ver 2.2.6. This might allow you to open, edit and save NEFs from your D4. If not, you will probably need to update to OSX Lion, 10.7.x, then update Capture NX2 to whatever the latest version of Capture NX2 it will take. I must explore this same territory.
I'm behind on my Mac OSX maintenance since I became sensitized to the risks of just simply applying the updates as they are offered by Apple. For even point rev updates to the OS there are considerable preparations which should be done, followed by some cleanup. I've drafted a 3 page procedure with about 48 pages of appendices to outline the process which I plan to use to update from OSX 10.6.4 to 10.6.8 ver. 1.1 via the Combo Update. I got side tracked into buying an iPad3 to read the draft procedure and appendices -- I don't want to print it since it evolves too much, and since the 48 pages of appendices include many links to other resources to handle possible problems.