After you get the National Pokedex, fly to Sandgem Town. When you go into Lucas's house(or Dawn's if you chose guy) there will be a little girl. Talk to her to find out who is swarming on that day(Swinub, Surskit, Likitung, Drowzee, Doduo, Zigzagoon, Beldum, Spinda, Makuhita, Skitty, Natu, Krabby, Corsola, Pidgy, Smoochum, Slakoth, Electrike, Voltorb, Tangela, Voltorb, Delibird, Farfetch'd, Absol, Spoink, Snubbull, Magnemite, Phanpy, Dunsparce, Nosepass and Cubone are all of the Pokemon that will swarm). Only one will swarm a day(all day) so you will need to wait 24 hours.
For this cheat you do not have to do that. Just fly to Sandgem and talk to the girl. If the Pokemon that is swarming isn't the one you want, then go outsiside and/or save it and shut it off. Turn your DS back on but first set your clock to 11:59 (23:59) and say yes. The DS automatically turns off when you say done so turn you DS back on and say play game. Then wait where you are untill midnight(if you are outside, then the lights in the houses go out at exactly midnight). Then talk to the girl and a different Pokemon will be swarming. You can keep doing this untill you find the Pokemon you want.
Have fun, Pearlprincess.
P.S. If you go straight ti the next day it will not work and the Pokemon that was swarming the yesterday will still be swarming.
Added by: pearlprincess Jul 30th 2007, ID#4153