I just found the answer to my question on the Nikon website. My works just fine when used as described below.
The built-in AF assist illuminator enables the camera to focus even when the subject is poorly lit. To use the illuminator, the camera must be in single-servo AF with an AF-Nikkor lens attached, and the center focus area must be selected or closest subject priority in effect. If these conditions are met and the subject is poorly lit, the illuminator will light automatically to assist the autofocus operation when the shutter-release button is pressed halfway.
For the AF-assist illuminator to function correctly, the lens must have a focal length of between 24 and 200 mm and the subject must be in range of the illuminator.