If your game is anything like mine, once you finish the story, the ghost studs will lead you right to it as if it were an active quest.
The ghost stud trail will take you to the Batcave and then into its Trophy Room. Once inside, run all the way to the right to find the set of a television show. The set itself looks like the living room of a house. The '60s Batman theme will be playing when you find it.
At the very back of the living room is a very tall bookcase. To its left will be a grapple point that you can hook onto (use Batman or Wonder Woman, for examples, and the B button should float next to them when they can interact with it).
When you pull it out, the bookcase collapses revealing two poles to slide down. Jump into the opening and you'll be taken to the Bonus Level. Complete the level and the achievement pops.
As an aside, in the same room with the bookcase, to the left hand side of the living room with the fireplace will be another grapple point that you have to construct after smashing some stuff. When you construct it and pull it down, the painting over the mantle releases Adam West.