there are a total of 3 chest to unlock this achievement
progress through the level until the final open canyon area where the return home portal is and you will unlock the ability to carry missiles. To launch a missile use
to fly high, then select
to look down and finally drop the bomb with
the first is back in the green box rooms which you will have passed earlier with what looks like a sun on the wall
the final 2 are in the open canyon area. one to the left on the ground floor as you enter the area and the other is directly in front of you on a higher ledge.
hope this helps