Instructions
Go to Sunnyshore City, located east of Route 222. Walk north on Route 223 until you reach the Pokemon League. Go inside and defeat the Elite Four to beat the game. You are returned to your house in Sandgem Town.
See each of the 150 Pokemon listed in the Sinnoh Pokedex. You do not need to catch the Pokemon; simply seeing them counts. Every time you see a new Pokemon in the wild or when fighting a trainer, it is recorded in your Pokedex.
Add Palkia to your Pokedex by going to Celestic Town, located to the east of Mount Coronet. Speak to the woman in the northernmost house, and she tells you about Palkia, adding its information to your Pokedex. This is the only way you can add Palkia to the Pokedex, since Palkia is not found anywhere else in the game.
Select the Explore Kit, given to you in Eterna City, to go to the Underground. Scan your radar for flashing orange dots. Head to the location of a dot when you see one, and tap on the stone walls around you to initiate a treasure-hunting game. At random, you may find an amber. Continue digging until you find one.
Leave the Underground with the amber. Travel to Oreburgh City by walking east on Route 203 or south on Route 206.
Give your amber to the scientist in the museum. In return, you are given an Aerodactyl.