How do you reach Celestic Town in Pokemon Diamond and Pearl?

Defeat the fourth gym leader. He is in Pastoria City. Then, go to where a Galactic Grunt stands. He runs away. Follow him everytime he runs away. Stop when you get to Cynthia. Answer, "yes" to both her questions and she will give you a potion. Go to Route 210 and use the potion on the group of Psyduck. They will scatter and you will be able to go through the path. Use the HM Defog and eventually, you will reach Celestic Town.

You can also use your Action Replay and select walk/run anywhere so you can go through the Psyduck. Warning: Action Replay May Freeze Up After 15-20 minutes.