You just have to press the button.
But it's a little difficult to get to it.
Everything that you can see on this map is a hint to something else that will get you closer to the red button. If its not a hint then its a red herring.
Make sure to use the texture pack provided (It should load automatically).
Don't cheat and play in adventure mode.
It is possible to finish the map without using all the hints and seeing all the rooms.