Dom is located on the roof of the tall circular building in Pillbox Hill, so you're going to need to either parachute onto it or use a helicopter. When you get there, you'll have to jump off the building and Dom will tell you to land on the flatbed of the moving truck below. It may take a few tries, so keep trying and don't give up.
If you've done the Drop Zone lesson for Flight School, then this is the same thing, except there's not much room to maneuver here because of the buildings. If you haven't done it, one of the easier ways to complete this is to freefall about halfway done before deploying your parachute. This should give you an easier time lining up with the truck, which completes the mission once you do land on it.
