I don't know if you've figured this out by now, but Theatre World is now most easily accessible from Square-Square World, which is in turn most easily accessible from a world called River Road, but not just any River Road, specifically the one you access from The Docks. If you don't know how to get to the Docks, you can get there from Apartments, then you can get there from the Library if you've been there before.
Another way is to go to the Spelling Room, which is accessible from Stone Maze, and put in "Urotsuk", to go to the Theatre World. You can also get there from Nail World, which you can get to from the Warehouse.