Instructions
Tap the "Pokegear" icon on the bottom screen of your Nintendo DS.
Tap the "Radio" icon.
Tap "Channel 2." This is Professor Oak's "Pokemon Channel," and he will detail which Pokemon is swarming and where. For example, he might say that a swarm of Yanma is on Route 35.
Travel to the area where the Pokemon is swarming. Walk in the tall grass until you find them. Pokemon that are swarming have a 30 to 40 percent chance of showing up instead of the usual breed of Pokemon found in that location.