The assembled ladder needs to be placed against the wall on the left side of the yard - just next to the bench. You'll get a hand cursor where it should go. Holmes will not attempt to climb the yard stairs if you are in the correct spot. Playing in first-person view might be a good idea here, if you are currently in third-person.
Also - be sure to have used the hammer on the ladder-with-nails in inventory, so that the nails are fully hammered in and not loose. Otherwise you won't be able to place the ladder against the wall.

If you still have problems, please let us know!