Instructions
Check the batteries on both the D40 and the external flash. Using an external flash takes more power than using the on-camera flash. In order for the D40's metering system to function properly with an external flash, both the camera and the flash must have a good charge. Replace the batteries if necessary.
Turn the camera and the flash off.
Slide the plastic cover out of the D40's hot shoe mount, located on the top of the camera, by pulling the textured tab gently towards you. This exposes the flash sync contacts.
Locate the hot shoe mount lock on the flash unit and set it to the open position. Align the flash mount to the hot shoe mount on the camera, then slide the flash's hot shoe mount gently forward to attach the flash.
Set the flash's hot shoe mount lock to the closed position to secure the flash. Power on your camera and the external flash to begin using the Nikon Creative Lighting System.