You need to ensure that external flash responded to the illumination of the built-in flash. Click the camera and go to "Custom setting menu".
Select "Bracketing/Flash" and in the opened menu, click on "Built-in Flash by selecting built-in flash.
Now, go to settings built-in flash, open the "Commander mode" to enter control mode and set the flash to work by installing a group (A) and the working channel corresponding to the channel of the external flash. For example, if you are using an external flash model Nikon Speedlight SB-600, your channel will be the third.
Tune directly to the external flash – press and hold simultaneously the "-" button and "zoom". Opens the settings menu.
Scroll through the menu items with the buttons + and -, and press "Off", which is accompanied by a zigzag arrow. Then, using the "Mode" button to select "On".
These actions you have enabled wireless contact flash and your camera. To exit the menu, again press and hold the buttons "zoom" and " - " or just turn off and then turn on the flash.
All settings are made – now your flash synchronized with the camera, and its screen should display information on the operating channel and group A. From this moment you can photograph.