Short level. Go to the far right of the courtyard and stand on the lone
block. Jump to make another block appear over your head. Hit this block and
a ladder will appear. Climb it up to the top, then leap to left to land on
the mansion's roof. Open the chest here for a Stop Watch. Then flip to 3D
and drop down at the right of the chest to fall on the balcony below. Open
the other chest here to get a Cursya card.
Now enter the mansion. Inside, don't get hit by the dogs or you'll be
thrown outside. Cross the hallway and go through the door at the end, which
you can see while in 3D. In the next hallway, climb up the stairway to get
on the floor above. Go in the third door. Go to the end of the room and
press the button. The ceiling will start falling on you. When it's about to
crush you, flip to 3D and jump on it instead. Then let it carry you back up
as it raises.
Head left and jump on the ledge above. Follow the path and use Boomer on
the yellow blocks to clear the way. Use the 3D to make it past the pillars
and you'll eventually reach a chest at the end of the path. Open it for the
House Key. If ever you want to visit the other rooms and fall in traps,
remember to use Boomer on the switches. Anyways, now that you have the key,
head back to the main hallway and go right on the bottom floor. Use the 3D
to get past the guard dog and use the key on the lock to free it. Now head
through the door and hit the Star Block.
Hope I've helped!
If you need more help, Ign wrote this great walktrough!
Just don't look into it unless you are REALLY stuck http://faqs.ign.com/articles/784/784008p1.html