How do you use itemfinder and how do you catch the item?

Well firstly you obtain the itemfinder from prof oak's aide if you have about 30 Pokemon (I'm not sure) in vermillion to the far east. (The guard house if i could call it that). Now a fine example of using it is when your in a underground tunnel. First you go on your items menu and just click itemfinder then use. It would respond either "Yes itemfinder is responding" or "Nope, itemfinder isn't responding". How to get the item is quite simple, just face the ground where you think the item is and hit a and you should get a item, unless your wrong. I think the itemfinder has a range of four squares around you (one square is one of your steps)