What Adobe product are you using to process your NEF files. There are a couple of Adobe products and some use Adobe's ACR plug-in or need a minor version update.
Adobe has released ACR, Adobe Camera RAW, version 5.5 plug-in for Adobe® Creative Suite® 4 or Adobe® Photoshop® CS4 software. If you Adobe product is an older version of any of those products, you can not install this plug-in.
The above links are for Macintosh and they point to the instructions and downloads. The instructions provide a lot of details about the updates and how to install the plug-in or product. If you have Photoshop Elements or Premiere Elements will need to scroll down to the link for instructions on how manually install the plug-in.
You could install Adobe's DNG Converter to convert the NEF files to the DNG format and then use your older Adobe product to open the DNG version of the file.
You could also use Nikon ViewNX to open the NEF and then use ViewNX's "File => Open with..." to directly transfer a TIFF image of the NEF with the camera settings applied to the image. You will need to use the most recent Nikon ViewNX.