Zia,
Go into your effects bin, video transitions>dissolves and drag a "dip to color" dissolve onto your edit point. Double click on one side of it, to open the transition editor. Set the duration for about 15 frames or there abouts depending on how quick you want the flash then click on the black box in the color parameter line. Drag the slider next to the color wheel, or any other method you choose to select "white" then hit ok. Play the edit and you should now have a white flash you can save in your favorites bin, to use again, by clicking back in the viewer and hiting option F.