Just download BaseCamp and use it to geotag the photos for you. I works by matching the time-stamp on the photo to your track data and then tagging the image file with the location data.
There is a connector on the camera marked GPS in and Nikon sells a GPS receiver that plugs in there.
My guess is that Nikon has a proprietary format so you have to buy their GPS.
The 76CSx can output data in Garmin (proprietary) or NMEA (standard) format via it's serial interface. It'd be nice if the Nikon could accept NMEA but you'd have to ask them.
BTW you have to be careful of the term "Geotag". The Nikon will store co-ordinates in the images EXIF data. In Garmin terminology "geotagging" means creating a waypoint associated with an image vs embedding the data in the image itself.
There are programs that will embed GPS data in EXIF fields. I use RoboGEO.
