Very easy. Seriosly, make one, like this:
1: make a tower of 4x1 flat bricks as tall as the thing you want to climb.
2: Turn collision, raycasting, and collision off on every one, except every 3rd one*.
3: Event one visible one as the following:
Onactivate-player-setvelocity-[0]
4: Make it look more ladder-y by adding 1x1x5 bricks to each side, until it reaches the height of the ladder.
5:You're done. Good for you.
*Here's a diagram, in case you don't get it:
___
___ <--visible
___
___
___ <--visible
___
___
**Change this number to fit the height you want it to launch you.