First, make sure that you have the 1-4 gym badges, and go to Pastoria City
Next, Talk to the Team Galactic Grunt who is standing next to the Great Marsh Observatory.
He will talk about a package, and run away.
Chase him, but prepare to battle your rival.
After beating your rival, keep chasing the Galactic Grunt to Valor Lakefront, talking to him when he stops.
After talking to him five or six times, he will finally give up and battle you with a Lv. 25 Glameow which should be easy to knock out.
Then talk to Cynthia in the black, and tell her youve seen the psyducks, and she will give you SecretPotion.
Next, go to solaceon town and travel north, to the Psyduck, and give the SecretPotion to them.
Then, Give The Charm Cynthia gave you to her grandmother in Celestic Town.