Catch 8 of the silver shining rabbits that randomly appear and the 8th one will give you the key. There's an easy way to do this that I found on GameFAQs:
Yoshi must enter the castle grounds through the front door. There are three rabbits at the end of the bridge and three more elsewhere in this area. If none of them are glowing then go back in the castle and out again and repeat etc. etc.
Unlock the car with the key in the door or with the remote.Unlock the car with the key in the door or with the remote.
With a key.
Key will turn but door won't unlatch to open.
Unlock it from inside the car with the door lock.
you use the key just like using the key for a door
you use the key just like using the key for a door
You have to get enough stars to unlock the star door(the one where peach is crying out for Mario) and when you beat Bowser you'll get a key for the basement type area. To get there you go through one of the doors underneath the stairs and go down the stairs there and unlock it.Then you get enough stars to unlock the next star door and then you beat Bowser again and then you will get a key for the door leading to the third floor,or second floor in your case.
Unlock the car with the remote or with the key in the door.Unlock the car with the remote or with the key in the door.
Unlock the car with the remote or the key in the door.Unlock the car with the remote or the key in the door.
If you mean that door in the rec room in Super Mario 64 DS with the number 8 on it, you just need to get 8 stars and then it will open. The course with Goomboss, who holds the key to the room where Mario is locked, is behind this door.
with a key
The Key of Destiny door cannot be opened.