MT Shooter wrote:
The D90 will use TTl quite well (better with an off-camera flash than the on-camera one). Meter the sunset, transfer those settings to the camera in manual mode, pop up your flash (or use the off camera unit), and shoot the scene, the flash will fill your subjects and the background will be properly exposed for the sunset.
That's what I would do. If you want it to look more light matched you need to gel with a 1/4 to 1/2 CTO on your flash.