Unmissable. When trying to get into your Mom's room to get the gift, after going into the back yard you'll need to climb the tree. Get the rope from the garage and use it on the branch.
Use the rope hanging down from the tree, Tommy will state that it won't support his weight. Now push the concrete block multiple times until it's under the rope. Use the rope then use the concrete block to attach them.
Finally, use the rope once again to climb up to the treehouse.