Eventually, a man wandering in the Mean Street South Area will tell you that he saw Pete ring the bell above the fire house once, then enter it.
Later, he will tell you that the clock above the train station needs to be set to 4:30.
So, oddly, at that point in the game Oswald can boost you up from the cargo in front of the train station, so you can reach the clock. Sometimes for me the clock is closed, and sometimes I have to shimmy around to the face, and spin, until the clock says 4:30.
Thin and paint and thin the firehouse so you can see and jump up and ring the bell that is way over the firehouse door.
Then, go to the balcony above the firehouse, and you will get the "enter" sign, to hold down the button that you use to enter a room.