Before Mission 7, select the demolition kit from the weapons menu. During the mission, after you kill the two brothers go across the bridge towards the rocket. When you get to the rocket, go left and down that road. When you get to the end of the road, there should be two doors (one on the left and another on the right). If you try to open them, they will say "secured". If you go on to a ramp that is on the right side of the left door, there will be a crack in the wall. Use the demolition kit to blow up the crack to get into the door.