Len is correct--the zoom is controlled by a sliding knob at the lower right of the telescope.
If you've shot everything you can see, I'd go over and check out that ladder--it didn't look to me as though it was down either, but it turned out that it was.
Are you sure you're looking in the right place? The ladder comes down to the left of the journey cloth that you reach by climbing the crates and jumping over the gap in the ledge...