Head to the southeastern gate in Hearthome City. When you try to leave the city, your rival arrives. Defeat his four-Pokemon team and go through the gate.
In "Pokemon: Diamond Version," trainers from across the Sinnoh region stop by Solaceon Town to visit the Day Care Center and Pokemon News Press. The Solaceon Town Day Care Center raises and breeds your Pokemon for you, and the Pokemon News Press manager rewards you when you show him rare Pokemon. Solaceon Town is first available after you get the Relic Badge in Hearthome City. To get there, follow the path on Route 209.
Walk south along the road and turn right at the junction. If you want to get the Good Rod, a fishing pole required to catch certain water-type Pokemon, speak to the boy on the far right side of the dock.
Cross over the eastern bridge and walk north across a second bridge. Turn right. If you have an Odd Keystone in your inventory, insert it in the gap in the pile of rubble next to Pokemon Breeder Jennifer. Doing so opens up a new area called the Lost Tower.
Continue to walk north on the path. Fight the Pokemon trainers on the path and, after you beat them, you reach the entrance to Solaceon Town.