Simply go to the level where you have to save the guys worker (Chad).

After you do that go up on the little balcony thing (make sure you get up on the ladder closest to the cage the worker was in).

On the other side you'll see the top of the cage below, on the right side simply look straight up and throw a grenade but make sure it's showing it will come down on you (just hold down the grenade button and throw it).

You will notice first it doesn't come down, it looks like it is flying and it blows up in mid air!