Go next to the two windows in the back where you go through to get to the crane.

You can see the side window, go outside to the ledge next to the side window. Go left as much as you can so you can see the side window.

Look two windows down and that's the window you must go to. Hold A then crouch, then look at the top of the pointy metal bar.

Then move forward and you'll be at the window, but activate the crane first.

Kill any infected that are clawing at you.