Instructions
Go to Route 110, a north-south route connecting Slateport City and Mauville City. Save your game and go to the long, narrow strip of land that is surrounded by water. Defeat your rival's three Pokemon. She will give you the Itemfinder as a reward for defeating her.
Press the "Start" button on your handheld. Go to "Pack" and use the D-pad to shift to the "Key Items" section. Highlight "Itemfinder" with the D-pad and press "A" to select it. If there is an item nearby, the Itemfinder will start beeping; and your character will change direction.
Head in the direction your character is now facing. Use the Itemfinder again. Continue using the Itemfinder, until your character rotates in place.
Move one square away from the spot. Use the D-pad to make your character face the spot where she was just standing. Once your character is facing the spot, press "A" to find the hidden item.