The multi-player room is in the Daily Paper building on Main Street.
It is the "You Know the Drill Eatery" on the middle level of the oil platform. (green door)
You enter the sewer through the restrooms in the park (boys use the boys' room, girls use the girls' room).
In Shark Tooth Island in the cave go to the right & down & then left and up there you are.
The room of requirement
The "valves" are the red control wheels in the sewers under City Park on Super Power Island. By turning the first one you find, you lower the water level. Turning one or two more will raise the level back up and you can reach the control room door. (Do NOTturn the one closest to the door or you will have to start this cycle over again.)
super power flashlight
turn off the power in rodrick's room.
There are three red wheels that lower and raise the water in the sewer. But the one you use in the Control Room is at the upper right, and you have to jump past three rats to reach it.
hit a ? box or go to the mirror room
You have to dodge Betty's green bombs for about 2 minutes, because you will slowly gain on her. Stay back so you have room to dodge. When you can finally get close, run into her and knock her down.
The ad-games almost always appear on the right side of Main Street. The multi-player room is the Daily Paper building.
Go through the mirrors in the mirror room!