A dowsing machine is an item you can use to find hidden items you normally wouldn't be able to see
You get the dowsing machine from Prof. Rowan's assistant on Route 207. You must first get a bicycle from a shop in Eterna City, BUT, before you can get the bike, you must take on the Eterna City Gym leader, and win the Forest Badge from her so that you can use the move Cut outside of battle. Then you must use Cut to cut down a bush in front of the Galactic Eterna building. Then go into the building and rescue the bike shop manager. Only then can you get the bike and then backtrack to Route 207 to get the dowsing machine from Rowan's assistant.
Hope this helps!! :D