Instructions
Load the game and travel to Pyrite Town. If you have not yet completed the story events here, do so. First, fight any trainers you encounter; then complete the Colosseum battles. Once you have done so, leave Pyrite town, and a new icon will appear nearby on your map.
Travel to the Rock Poke Spot. You will find Duke and a reporter on the scene, talking about the discovery of wild Pokemon. Talk to Duke, and he will ask you to help demonstrate Pokemon Snacks and Poke Spots. Step between him and the reporter, and they will give you your first set of Poke Snacks.
Follow the on-screen instructions to leave snacks in the Poke Spot and coax a Pokemon out of hiding. Duke will also give you the Spot Monitor to tell you when something is in the Poke Spot enjoying the snacks. When you have finished this event, return to the map.
Speak with Duke when you run out of Poke Snacks, and he can sell you some more.
Check the PDA e-mail after each new story event. You will randomly receive an e-mail telling you that Poke Snacks are now available in the Poke Marts all over the world. From this point forward, you can purchase all the Poke Snacks you may need at any Poke Mart.