Go on down the ramp (leaving the warthog if you brought it in) and kill the group of Covenant at the bottom by the door. Head in (I suggest taking the warthog into the first area to make it easier on the higher difficulties) and eliminate all enemy presence. This is the entrance to the map room Cortana has been talking about. Keep going and you'll fight a pack of Elites and a bunch of Grunts at the entrance to the building here. You'll see a steep hill ahead and when you get to the top you see a Covenant dropship fly off.
