First simply equip it so that you see the circular grid screen on the touch screen of your DS. Then use the stylus to highlight areas of the grid. When you see areas of the grid change from red to orange to yellow, then you know there is a hidden item in that direction. If you are close enough to the item, it will appear on the grid as a orangish dot inside one of the squares of the grid(hte dot only appears if you highlight it with the stylus so look at the colors). The small square in the center of the grid represents the player avatar, so by figuring out which direction the dot is in relation to the square, you should be able to locate the item.
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In Pokemon Black, the Dowsing Machine is automatic. When an item is in range, it will point in that direction.
Go to your bag (by pressing the X button) and select Key Items and select the Dowsing Machine. The dowsing machine will tell you where to go by pointing out a direction.
The purpose of the dowsing machine is to find things. when you hear a beep, your close to a hidden item. when your on the dot, press A to find the hidden item.
YOU HAVE TO PRESS THE BOTTOM SCREEN ANYWHERE(WITH THE DOWSING MACHINE ON THE BOTTOM SCREEN) YOU HAVE TO PRESS THE BOTTOM SCREEN ANYWHERE(WITH THE DOWSING MACHINE ON THE BOTTOM SCREEN)
Use the Dowsing Machine on Route 20. You'll eventually find one.