Instructions
Spot every Sinnoh Pokemon to complete the Pokedex. Have Professor Oak upgrade the Pokedex to include Pokemon from other regions like Hoenn or Kanto. Meet Professor Oak at Pal Park, located on Route 221.
Listen to Professor Oak explain the Pal Park. Save the game and power off your Nintendo DS or Nintendo DS Lite. Replace the black plastic GBA cover with the "Pokemon Ruby" cartridge.
Start "Pokemon Ruby" and deposit Latios and five other Pokemon into your Pokemon Storage System. Save your progress and turn off the hand-held. Transfer Latios and the five Pokemon from the "Migrate" option on the main menu of "Pokemon Diamond." Continue from the last save point in "Pokemon Diamond." Enter the Pal Park facility.
Speak to the man wearing a yellow hat and green clothing. Say "Yes" to participate in the Catching Show. Find the Latios in the Field. Look for the other five Pokemon in the Ocean, Pond, Mountain, Forest and Field. Catch the imported Pokemon with a Park Ball.
Access the Pokemon Storage System from a Pokemon Center. Withdraw the Latios from storage to your party of Pokemon.