There's a small keyhole on the wall near the entrance to the cab. You need to unlock the doors using that keyhole first. And after that you use key in another keyhole over the doors to open the hatch with buttons.
Yes, everyone seems to forget that key slot on the back wall to activate the door control panel above the doors. I only knew how to operate the guard panel on the pacers due to being around them quite a lot for 9 months.
The sort of thing that should be included in the instruction list, given it includes things like "Turn the master key to on" and "Set the reverser to forward"
