What isn't readily explained in that tutorial (It took me a long time to make a ladder from that tutorial) is that when you place the func_useableladder, you need to grab the little white circle/dot and drag the SECOND box (that is perfectly on top of the FIRST box in all three views) to the top of the climb. It's a two-part single entity.
Also, ladders work poorly in Deathmatch. It's like you've developed a severe hiccup as you climb.