Super Mario 64 DS

There are two requirements to find the ? Switch. First, you need to have 14 Power Stars. Second, you need to have Mario unlocked.

Using Mario, go to the Castle Lobby. Directly in front of the small staircase, there is a sun pattern on the floor. Stand in the center of this pattern and set the camera to first-person mode (use X if in Standard Mode). Look up to the source of the beam.

You should now be transported to a new stage, the Tower of the ? Switch. To find the ? Switch, simply fly to the low tower (the only one you can land on) and it will be right there on that platform with you. From there, all you need to do is push it, and the ? Blocks will be unlocked.