How do you get to Dialga in platinum?

Answer 1

You'll need to defeat the Elite Four and Champion Cynthia, caught Giratina, then talk to Cynthia's grandmother in Celestic Town where she'll talk about summoning Dialga and Palkia. After she's done talking, you'll need to go through Mt. Coronet and reach to the place where you are nearly at the peak of the mountain (where there is snow), then use Rock Climb up a slope and enter a cave entrance. Look for the water, then use Surf and Waterfall to reach the top to see another entrance. When you go inside you'll find the Adamant Orb and the Lustrous Orb. After obtaining them, go to Spear Pillar, where you met Giratina and Team Galactic at first, and a blue and a pink portal will be floating at the corners of the place. Make sure to save before you interact with the blue portal, where Dialga will come out to attack you.