Instructions
Head to Celestic Town, on the eastern side of Mount Coronet. If you have already been to Celestic Town, you can get back quickly by using a Pokemon's Fly move.
Go to the large house in the north of Celestic Town and talk to the Elder there. She will tell you more about Dialga and Palkia and about the Adamant and Lustrous Orbs.
Head back up Mount Coronet. After using Rock Climb to get higher up the mountain for the first time, at the first snowy area go right, towards the waterfall.
Use a Pokemon's Waterfall move to climb up the waterfall and collect the Orbs from the top.
Head back up to the top of Mount Coronet and Spear Pillar and you will find a blue portal.
Go through the portal to fight Dialga, who is a Level 70 Legendary Pokemon, so be ready for a difficult battle.
If you fail to beat or catch Dialga at this point, go to the Pokemon League and defeat the Elite Four, after which Dialga will re-appear at Spear Pillar.