Due to my current resources i cannot provide an example, but you can make a simple ladder by adding an object then have that object check if the player is touching it, if that returns yes, make it check if a key of choice is pressed, if not maybe show a Ui item that tells the user what to press, if the key is pressed, simply increment the camera.y value for first person games, and for 3rd person simply increment the model acting as the player character.