How do I make Dialga and Palkia appear at the top of the Spear Pillar?

Well if I'm not mistaken first you need to go to Celestic Town. There you need to talk to Cynthia's grandma aka the town elder. She should show you an old book shes found, after that head back to Spear Pillar. There should be a portal, go up to it and press A. That should start a battle with either Dialga or Palkia. After you've caught which ever legendary you battled, head out and then come back. There should be another portal there. Click on it and you should battle the legendary thats left.

Alright, first defeat the Pokemon League even if you already did, then go to Canalave Library. Lucian should be at the first floor. She will have a talk with you and leave. Go up to the first floor. Read the book titled, "The Original Story". After this get the National Pokedex after you meet every Pokemon in the Sinnoh Pokedex and talk to Prof. Rowan. Now go to Turnback Cave and find three pillars and when you find the third pillar go in any room and Giratina will be in. Catch/defeat him by talking to him. They should be there along with Mesprit, Azlef and Uxie in their caves.

Mabe I'm mistaken but mabe you got the rong orb because the orbs you need to find are called a adamant orb and another orb for palkia called the lustrous orb collect them and go to sphere pillar and you will see one blue hole click on it and you battle dialga then exit sphere pillar and then enter it again and you will see a pink hole do the same as last time okay