How do I turn on the Rear Command Dial in my Nikon D80 Digital Camera?

That depends on what you want the command dial to do. If you're in either the Manual (M) or Shutter Priority (S) modes, the command dial will change the shutter speed. If you're in the Program (P) mode, the command dial will shift the program exposure. If you're in any other exposure mode, the command dial does nothing.

If you hold down the metering mode button, the command dial will change the exposure metering mode.
If you hold down the exposure compensation button, the command dial will change the amount of the exposure compensation.

If you hold down the release mode button, the command dial will change the release mode.

If you hold down the AF button, the command dial will change the autofocus mode.

If you hold down the QUAL button, the command dial will change the image storage mode.

If you hold down the ISO button, the command dial will change the ISO.

If you hold down the WB button, the command dial will change the white balance setting.

If you're looking at the menus, the command dial will navigate the menu.

If you're reviewing pictures, the command dial will move to the previous or next picture.

Hope this helps. If you still have problems or other questions about this, please don't hesitate to reply to this post.