Where do you find the key to the door in the middle of the second floor..

If you mean directly up the stairs in the main entrance, then you have to beat Bowser in the Fire Sea. That's the one in the basement, near the entrance to Dire, Dire Docks. Once you defeat Bowser and grab the key, you should be able to go through the door.

In order to get the key to the door directly in the middle of the irst floor, you have to go into one f th ewooden doors on the first floor. Then you'll notice that completely opposite of where the Boo is floating, there are red stairs going down. Go down and through the door. To get that door's key, you have to defeat Bowser through to star door that is on the econd floor. Any how, you go through that door and you'll find yourself in a small room with a passageway to the right. Go through that passage way and into the door. Then there's ANOTHER tunnel. Turn to your right and keep going straight. Soon you'll see a big painting of a fireball. Jump in and then after a few levels, you'll have to defeat Bowser. Defeat him and you will get the key. Good Luck!