From the Nikon site, do you have one of these flash units.
Support for the Nikon Creative Lighting System when using SB-900, SB-600, or SB-400 Speedlights, or the SU-800 Wireless Speedlight Commander
If so You set the flash mode to "comander" the pop up flash must be UP
This Video by Nikon shows how but you will have to read the manual to get a full description go to the flash section http://www.nikondigitutor.com/eng/d300s/index.html
Then look at them all :)