How do you get Seel egg in Pokemon Diamond Pearl Platinum?

You have to put a Dewgong in the Pokemon Daycare in Solaceon Town with a Ditto or another Pokemon in the same breeding group and walk until they tell you that they found an egg. Then you can hatch the Seel by walking 5,355 steps, or you could have a Pokemon in your party with the Ability Flame Body to cut the number of steps in half.

Pokemon that have Flame Body: Slugma, Magcargo, Magby, Magmar, and Magmortar.