If you want to use and control an external flash unit like SB-600, SB-800 or SB-900 through wireless media using your Nikon DSLR camera built-in flash, you have to setup a commander mode in your camera. If you have already setup commander mode, you can follow the steps below to setup your flash unit.
How to setup SB-600 as a Wireless Remote Unit?
1. On the SB-600, press the ZOOM and "-" buttons simultaneously for 2 seconds to display the Custom Settings mode. You will see OFF displayed by default.
2. Press the ZOOM or MODE button to select wireless mode ON. Now you have activated wireless remote mode.
Next step is to set the Channel and Group as per your camera settings.
3. To set the group and channel, press the ZOOM and "-" buttons simultaneously for 2 seconds. You can also alternatively press power (ON/OFF) button for one second. Default settings for Channel and Group are CH 3, Group A.
4. Press the MODE button to highlight the Channel (1, 2, 3 or 4 ) and it will display the blinking channel number.
5. Then press the "+" or "-" button to set the Channel number and press the MODE button to lock the selection and move to the Group display.
6. When you see blinking Group displaying on the screen, press the "+" or "-" button to set the Group and then press the MODE button to lock the selection.
Note - Please note that the Channel number and the Group settings should match with your camera's commander mode group and channel settings. Now you are all set to use SB-600 flash unit remotely to be controlled by your camera's built-in flash unit. But please remember to pop up the camera's built-in flash so that it can send the signal and the exposure information to the external unit, SB-600. Otherwise the setup won't work. If you want to turn remote wireless mode off after getting the job done, simply follow the last two steps.
7. Press the ZOOM and "-" buttons simultaneously for 2 seconds. Then the screen will display ON (Which we did on step 2).
8. Press the MODE button and the screen will display OFF. Which means wireless remote control is off.
If you are looking for how to configure a Nikon SB-800 and a Nikon SB-900 flash units in a wireless mode, here are the links for How to configure a SB-800 flash unit as a Master flash unit and How to setup a SB-900 flash as a Master flash unit.