Second, to my knowledge, with the SB-600 wireless all you can do is TTL. I have never been able to find a way to kick it into manual mode when in wireless mode.
You set the Group A or B Mode to "M" and Comp can be adjusted between 1/1 to 1/128 in the Commander menu in camera. I find it useful with multiple flashes having the SB600 as a kicker/hairlight/rimlight with reduced output.
Another tip is to add the Commander mode to My Menu. I can't remember off-hand if the D300 allows you to set your Function button for quicker access, the D700 does allow.