You complete your sinnoh pokedex, then go to your hometown and get the national 'dex then go to pal park and migrate pokémon from ruby, sapphire, leafgreen or firered.
Beat the elite four then if you did'NT see a drifloon.ON FRIDAY go to the valley windworks on that day then you see a drifloon talk to it and use a fast ball to catch it and then talk to professor...