Heres how it works, you have to find a guy with a gas mask, hes probably somewhere in a place called Louisa, then you will get attacked by a dino, throw a limb at it and it will go away(if it doesn't, you're screwed). Then, go look around for a place called Wilmosaum(or something like that) and put on the mask. Keep going forward and there will be the ladder.
For all who can't use the rifle, its because you need bullets, you can find them separately. The flare doesn't do anything, it just distracts the dinos long enough for you to find a limb. I'm not sure how many bullets it takes to kill a dino, it depends on where you shoot.