Instructions
Beat the Elite Four in "Pokemon HeartGold" or "Pokemon SoulSilver" and go to the Silph Co. building in Saffron City.
Talk to Steven in the building's lobby and choose the "BLUE STONE" reply. He will give you a Mudkip as a gift.
Transfer Mudkip to "Pokemon Diamond" by using the trade function in any Pokemon Center. Using two Nintendo DS systems, go upstairs in a Pokemon Center in both "Pokemon Diamond" and the copy of "Pokemon HeartGold" or "Pokemon SoulSilver" where you have Mudkip to trade it to "Diamond."
Capture a wild Ditto in "Pokemon Diamond" by using the PokeRadar item on Route 218. After using the PokeRadar item in tall grass, sections of the grass will shake. Run to those sections of grass for a chance to catch a Ditto.
Bring Mudkip and Ditto to the Day Care Center found on the left side of Solaceon City. Give the pair of Pokemon to the Day Care couple. Check back after taking 256 steps to retrieve the Pokemon egg that Ditto and Mudkip created. If there is not an egg waiting for you, walk another 256 steps and check again.
Walk with the Pokemon egg in your party and Mudkip will hatch. It takes between 1,000 and 10,000 steps for a Pokemon egg to hatch.