What you need to do is if you have played pokemon diamond or pearl you have to go east from the place you got it in diamond/pearl Go to the mt.coronet entrance and dawn or lucas will give you the vs seaker+ the dowsing machine
Where Do I Find The Poketch App Dowsing Machine In Pokemon Platinum?
After you do the eterna city badg you will find dawn che gif you a vs seeker and the dowsind app
Its at the top left
You get it in mt coronet enterance oreburgh city side