Its pretty simple just press shift and you can climb on it
If you're talking about the first IGI, then the default action button is the Shift key. Check your button config in the main menu to find out what the action key is. Stand facing the ladder (perpendicular to the wall) and press the action key to grab onto the ladder. Then hold the forward key (probably up arrow by default but I changed this to "w" when I was playing) to climb up and the backward key to slide down). It isn't like half-life where you just walk into the ladder to start climbing it.