My Sim wouldn't go up the ladder at first either, just waved her hand with a routing error. She was on the first floor so I went up to the second floor to check the top of the ladder wasn't blocked, nothing there, selected the ladder from the second floor for her to climb and she went up no problem. She's used it up and down fine ever since. Maybe that first routing is a bit glitchy but once you get them to use it once they seem fine with it.