all you really have to do is go up to an invisible Pokemon (keckleon, kekleon? forgot the spelling of it) and once your in front of it, press "A". The Pokemon will appear and either battle you or flee.
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You can capture one in the wild west of Lilycove City. It is rare. Or, if you have a Devon Scope, you might get blocked by something invisible. Use the Devon Scope, and it should be a Kecleon. There are a few around Fortree City.
You have to find Steven and he'll give you Devon Scope and you use that on the invisible object.
you have to find steven get a Devon scope and use it on the invisible keckleon in front of the gym
u don't go to the bag to use it. You press the a button,then the game ask u to use the Devon scope say yes then the object becomes visible. to get the scope: talk to Steven on the bridge on route 20.catch or defeat kecleon then he gives it to you.
when you rescue the Devon Goods, the person will bring you to the president. He will give you a pokeNav. As you walk outside the building, a scientist will upgrade your pokeNav. Use your Pokemon and there will be a setting called "Hoenn Map"