The Itemfinder only helps you locate hidden items, it doesn't allow you to dig for them. To dig for items, you need to learn the move "Dig" from a Pokemon that can learn it.
Here's how to find hidden items:
1. Use the Itemfinder: This will indicate the general direction of a hidden item nearby.
2. Walk around: Move around in the area where the Itemfinder points you. You'll hear a sound getting louder when you're close to the item.
3. Look for a shiny spot: The hidden item will be marked by a shiny spot on the ground.
4. Interact with the spot: Press "A" to interact with the shiny spot and uncover the hidden item.
Remember, you can only use the Itemfinder once in each area before it needs to be reset.