Swarms are another feature that have been retained in the change from Diamond & Pearl to Platinum. Swarms remain the same and require you to have the National Pokédex. Once you do, go see your rival's sister in Sandgem Town. She will tell you where the swarm is and what Pokémon is there. Once known, you can go there and find out. The swarms change every day so always check so that you can get the Pokémon you want