Instructions
Go to Route 225. Route 225 is located between the Survival Area and the Fight Area on the northeastern island in "Pokemon Diamond."
Walk north along the path and stick to the left side of Route 225. Look for the house placed at the base of the steep cliff face.
Take the left path past the house and climb the stairs on the cliff. Walk through the grassy patch and use Surf to cross the small lake.
Pick up the Dubious Disk at the bottom of the stairs on the other side of the lake.