hmm, you know, there is a tutorial for it.
[url=http://cariad.co.za/twhl/tutorial.php?id=34]Clicky
but anyway, first make the Brush with the {ladder texture a func_wall.
Set the "Render mode" to "solid" and "render amount" to "255".
Then make a brush next to it(the actual ladder) and make that func_ladder.