Where Is Dialga?

For Diamond, you have to beat Cyrus then he'll give you a Master ball. After that, go to Mt. Coronet and beat all the Galactic Grunts there. When you are at the top, go
through the entrance to Spear Pillar. After that, you have a tag battle
with Barry against Mars and Jupiter. Beat them both, then get to Cyrus
on your own and defeat him. Heal up, then face Dialga!!!!!
In the battle with Dialga, you can use your master ball(it has an infinity chance of catching) but there is only one master ball, so I would hold onto it. But you choose. DON'T DO WHAT I DID:
I thought there were more than one master balls when I was younger, so I used my Master Ball on a Unknown. After you capture Dialga, go to the end of the Spear Pillar and get the Adamant Orb (when Dialga holds it, it gives it more power). Hope this helped.

In pokemon Diamond you have to beat galactic boss Cyrus. After he gives you a master ball go to the top of Mt.Coronet (Spear pillar) and beat everyone up there. Then you will have the chance Catch dialga or defeat him.(Us the master ball at the begining of the battle like a quick ball). In platinum deep inside Mt.Coronet you will find a Lustrious orb and that summons dialga to spear pillar. Hope this helped!!  

On the top of Mt. Cornet, after you beat Cyrus and all of the team galactic commanders.