Help with flash syncing

[KmH]

Yes, I was able to mount my flash to the camera and use my max shutter speed of 1/4000.

You can do it off the camera using CLS too, if you put your D90 pop-up flash in commander mode. commander mode is covered in the D90 users manual and there is a good book The Nikon Creative Lighting System by Mike Hagen that covers how to use CLS and the SB-600, SB-800 and SB-900.

Since CLS is an optical system (infrared) reliability can suffer when it's used in direct sunlight.