Go to the crane, you will see a little 'Control Room' on the main stalk of the crane, just below the part you normally walk on. Climb on the crane from the yellow platform. Turn left when on the crane and get above that control room. Get your guy so he is standing on the edge of the crane, just above the slanted glass of the little control room (facing the 'Kriegler' sign). You should be looking at a yellow support bar that is almost vertical (as opposed to the slanted ones). Slowly back off of the edge of the crane. As soon as you fall off, hit A (you may see a quick flash of 'hit A to climb'). If done correctly, you will catch the edge of the control rooms slanted front and your guy will climb onto it.

There is no way out of this spot except death (or the game ends), but people won't expect you to be there when they scan the crane and so they may not see you. Try dropping a Tactical Insertion before you drop off, just in case you miss it.