When you enter the Rocketyard, you want to go to the "middle" section, where a couple Hive and Fallen are fighting each other. Look for this truck on the left. Once you clear the area, you're going to jump on this truck and walk to the very end of the trailer. Facing toward the back of this section, turn right.
Across the ditch, or trench, or whatever you want to call it, and on the wall there is a door. This is where the Fallen spawn from. Its usually one Vandal and a couple Dregs. Hard to see from this picture, but its tucked away in the corner in the shadows. On top of the truck is the best spot to watch the door, but you can also stand on the rails behind the truck in the previous picture.
Here's a little zoomed in picture to show you where it is.