Put your SB-600 into remote mode:
- Power up the Speedlight.
- At the same time press ZOOM and the - button for 2 seconds or more.
- Press the + button till you see the sharp curve symbol with the arrow head on the bottom and pointing right (#2 on pg. 14 of the manual).
- Press the MODE button till you see ON.
- Press the ON/OFF button.
- Press the MODE button to navigate to CHANNEL
- Press + or - to select the same channel you have set in the Commander mode of the camera. If the SB-600 is the only speedlight you're wanting to control, use Channel 1, group A.
- Press the MODE button to navigate to GROUP.
- Press the + or - button to select the GROUP the cameras Commander is set to.
- Make sure the light sensor window for wireless remote flash on the SB-600 is visible from the camera position.
- Make the Image.
Have another look at your SB-600 users manuals "Advanced Operations" section beginning on page 58.
There are several books available that describe the Nikon Creative Lighting System in a more palatable way than the manual does. Look for the one written by Mike Hagen. It's pretty good.