Hi Rick,
On the D700 (which I imagine is pretty similar to the D300) one has to go to the Custom Setting Menu [pencil icon] and select 'e' Bracketing/Flash submenu and then option 'e3'. Change this from TTL to C. Flick up the pop-up flash.
Then on your SB600, hold down the 'Zoom' and '- (minus)' buttons until the display changes. Press +/- until you see the symbol for a squiggly line which indicates CLS triggering. It will probably by default be 'Off'. Press Mode to switch this to 'On'. Press 'Zoom' and '-' again simultaneously and CLS should now be active.
If this doesn't make sense, please drop me a PM and I will be happy to provide my mobile no and will talk you through it.
Tim