Notes to myself. These were copied from Ken Rockwell and Flickr.
Use the CLS on the D90:
Move up or down to: PENCIL menu.
Move right and down to: e Bracketing/Flash
Move right and down to: e2: Flash cntrl for Built-in Flash
Move right and down to: C Commander Mode
-- (if u dont want it to contribute to the image)
Further click to the right to set the commander mode to "TTL."
You also may set the commander mode to Manual or AA, which are other modes to fire the remote flash. The SB-600 only works with TTL and Manual. You set the manual power level at the camera.
(Default for the SB-600 flash is channel 3,)
Don't forget to POP UP THE BUILT-IN FLASH!
On your SB-600:
Hold down ZOOM and "-" together to enter the CSM settings. That's why you see a gray "CSM" marking between those two buttons.
Press either the + or - buttons until you see an icon that looks like a wiggly Z-shaped arrow. This arrow refers to wireless communication between the flash and camera. When you see the wiggly arrow, press MODE to make it say ON.
Press ZOOM and "-" together to get out of the CSM mode. Even easier, just tap the power button to get back to normal operation. It won't turn off if you hit it while in the CSM settings.
Presuming you did this correctly you'll see "CHannel 3" and "GROUP A" displayed. If you see different channels or groups then press MODE to get one or the other to flash and then the + or - buttons to set them back to 3 and A.
The other channels and groups are for people who want to try to rig up a zillion flashes to work together and control them all separately and remotely.
The SB-600 doesn't go into standby in this mode, so you can run down your batteries if you forget and leave it this way. It just sits there blinking its little red LEDs visible from the front.