Place the following items on the blue and green buttons. Then head over to open the switch to get the starite.

Round 1: Rock
Round 2: Stone
Round 3: Statue

Scribblenauts Remix 6-8 Cheats Hints gallery:
6-8-1-a: The Starite is close, but how can Maxwell get to it?
6-8-1-b: Weigh down the green and blue buttons with objects.
6-8-1-c: Weigh down both buttons, and open the last door with the red switch.