1. Go to the "View" pull-down menu at the top. Check "Show Tile Bounding Box".
2. Tiles for ladders are at Generic Tiles/Ladders. The segments show up very small, so in the "Tile List Scale" field change the number from 0.5 to 5 and hit enter. Pick the segment you want to use.
3. If your ground level is at 0 and your roof level is at 13, then start at level 13. Align the first segement next to the roof and click the left mouse button to position the segment. From now on be careful not to shift the mouse from that position.
4. Hit the Page-Down key to move down a level and gently hit the left mouse button again. Keep doing this until you get to level 1. If you have a mouse or keyboard with a thumbwheel this process will be much easier.
5. Go to the "Select" field at the top and while holding down the shift key, left click each segment and Page-Up to click the next segment. Do this for the whole ladder and the bounding box will have a blue tint to it.
6. Go to the "Tools" button and click the "Climbable" option.
7. That is all - it should work perfectly. If not, practice a few times.