I think there's a couple different ways of doing it. The way I'm doing it, is climbing on the wall, directly to the right of the camera. Tap D, or whatever you move right key is, and then jump toward the building again. I think it's a timing thing, but if you do it correctly, you'll be on an invisible wall to the right-side of the main wall. Tapping W puts you into the building.

You might need to mess with this a bit though. I hope this helps.