I think you just have to pay around 5 Star Coins to get in. If you're talking about the one at the bottom, you'll need to go into the first Ghost Mansion. At the start, hit the leftmost block to reveal a Red ? Switch and hit it to reveal 5 doors. Go into the bottom-right door. Next, hit the invisible block that the hands at the top of the screen are pointing to to reveal another Red ? Switch. Take the platforms up and go into the door on the left. In the next room, hit the P Switch to make all the coins turn into blocks and go left until you see 3 lone brick blocks. Stand still and you'll see the third block from the right of a long line of blocks shine. Hit that to reveal a Golden Piranha Vine, and quickly make your way up to it. At the top of the vine is a door. After you go through the door, you will complete the secret exit of the level, which will reveal the path to the Green Mushroom House.
I hope this helps!
