Hi

The USB port on the camera is a dual purpose port and its functionality depends on the supplied cable you are using.

Assuming your are connecting using the supplied USB - USB cable and not the USB - Video cable and you are using Windows 8 or 8.1 see this article.

If you are using Windows 7 did you install the Nikon software that came with the camera?  If not do so - this should install drivers for the camera so Windows will recognise it.

When plugging in the USB to camera and computer make sure the camera is turned off, once connected to the computer turn on the camera - Windows should then detect it.

Also try this:-

On the camera itself, unplug the USB and turn it on

1. Press MENU
    
2. Press ▲▼to choose SETUP...press OK
    
3. Press ▲▼to choose INTERFACE...press OK
    
4. Press ▲▼ to choose USB...press OK
    
5. Press ▲▼to choose MASS STORAGE...press Ok
    
6. Press MENU to exit
    
7. Then plug it back into the USB cable.

HTH

John EB