The func_ladder object is supposed to be invisible. All a func_ladder does is make a 'zone' where you can climb. You can't see it.
In order to tell the player "HEY YOU CAN CLIMB HERE" it is helpful to put something that LOOKS like a ladder there too. You can make this part however you want. My recommendation is here:
http://www.superjer.com/learn/func_ladder.php
