That map is crap and should not be in rotation -- it's not even lit.
But anyway, as mentioned before, the trick is the "double jump." Jumps add to your upward velocity. The more jumps you do, the more your upward velocity increases. The easiest way to do it, I think, is this: Press the jump key quickly, lift up, then press and hold it. That way, when you touch the edge of the box, the jump key is already pressed down and you don't have to time it perfectly. Just start your own game on the map and practice it over and over. You'll get it eventually.