It is located near Damsen City, where it is grassy and there are rock pillars. You will find a place called "Odd Headquarters". Try hitting him. When you go down stairs, you will see a robot that is destroyed. Electrocute him with an Electel and he will become movable. When you go under, you will be in a place titled "Black Market". You can get items from the man near the counter that you cannot find anywhere else, such as Revival Fruit. Find some gems and turn them to a solid block by using the Change Rod. Walk on the blocks to get to a fence-like area. Use the Mist Jacket to move through it. After you are, done hit the switch and get the treasure chest.