Do you have the ipod cable? Have you done the bypass mod?
If you have the ipod cable, make sure that the TV Out setting in the Ipod is set to either ask or on. This is found in the video section under Video Settings. Now on the D3, make sure you are under the Ipod section. Look in the upper or lower right corner for the "Video" button. Press it. The D3 will show a black screen with instructions to start the video from the ipod. Start your video and enjoy.
You can fast forward and rewind from the D3 screen during ipod video playback, but you cannot start a video from the D3.