On the 1st level that you use demo charges, kill everybody at the end
of the level except the guy in the treehouse. after a while he will
come down then kill him. go in the building across the one with the
explosive. when you walk in you will see 2 boxes. DO NOT BREAK THEM!
look up and you will see a hole. run and jump(it took me a few tries)
on the box to the far left. run and jump again toward the hole. and
then climb. and boom.