Instructions
Walk down Mean Street and speak to Oswald. He will inform you that the projector to Tomorrow City is broken, but can be fixed with five power sparks.
Visit various shops and locales on Main Street if you do not have enough power sparks yet (you will have already collected several earlier in the game most likely). Speak to The Usher and Horace of Horace's Detective Agency to nab a couple of power sparks. You can also buy a power spark at the Emporium with E-tickets.
Talk to Gremlin Markus and gave him the power sparks. He will use them to fix the projector screen.
Jump through the screen to reach Tomorrow City.