Leave the trailer and look at a ladder on the side wall. It's attached only by a rod. Cut it with the saw to take a small ladder.
Do the same with an identical small ladder on the middle trailer.
Go to the main road and head for carousel. Go downstairs on a round place, circle the carousel and head to the Haunted House (merry-go-round should be on your left; while passing it, Watson will notice a control panel).
Getting under the Haunted House, climb up under the door. Holmes will hear that someone is inside.
So march on the left side of building and notice a window up on the floor. Join two ladders and wire in equip and make a one big ladder.
Set it under the window and climb up.
It turns out that you have to cut bars. So do it, by clicking a saw at four window's corners.
When the bars fell off cut a middle glass pane with the diamond ring.
Pull the cut piece of pane, and open window through the opening. Detectives spread: Holmes has to look for telephone in the Haunted House, while Watson sneaks to the anarchists' workshop.