Where is the glitch on fullmoon island in Pokemon diamond?

You don't really need a glitch. After you beat the sixth gym, (check and see because i haven't played in over a year) talk to the sailor guy and he will say his son is having nightmares and he needs the feather to wake him up, so he takes you to full moon island and you see cresselia. It flies away and you see the feather. Press a on it and you will obtain it. Take it back to the kid and he will wake up.

if you meen darkrai, go to fullmoon island use walk through walls to go right (the way) don't stop until you see another fullmoon island darkrai will be in the passage.