Here's how you do it. After you obtained the national pokedex, defeat the champion. Next go to your rival's house and talk to her sister. If she didn't tell you anything about swarms, save in front of her, turn the power off then play the game again. If you did this a few times and it didn't work, you'll have to wait until tomorrow then try again. If it still didn't work, that means there are no swarms for the day.
Hope this helps!^^