There is quite nice looking tutorial of adding ladders to game. I have not yet tested these instructions but they seem rational enough.
See: Simple Ladder for Platformer Game
Tutorials > Beginner > Theory > Simple Ladder for Platformer Game
Since I can't post links, I don't.
(Edit.. there's another tutorial too) I have not tested this one so I can't tell if it works or not.
Tutorials > Intermediate > Tips-and-Tricks > A Simple Method for Climbing Ladders in Platformers