How to Get Dialga in Pokémon Diamond

Defeat the first seven Gyms. After you defeat the seventh Gym, Cyrus will have captured the Lake Trio (Azelf, Uxie, and Mesprit) and taken them to the Galactic Building in Veilstone City to extract their red gems to make a red chain in order to summon Dialga.

Go through the Veilstone Building. You'll need to battle any Galactic Grunts on your way until you find your way to the lab where Cyrus is. Battle and defeat Cyrus. He will tell you to go to the warp panel that leads to the Lake Trio and to save them. He'll then give you a Master Ball, then head to Mt. Coronet to finish what he started. Locate Saturn and he'll challenge you to a battle. Press the button to free the Lake Trio after defeating him. They will float off from their prison and Saturn will flee to Mt. Coronet. Take the green warp panel to get out of the building quickly.

Head to Mt. Coronet. Don't be confused by the maze-like format. You need to find the right way, defeating the Galactic Grunts along your way there. You need Surf, Rock Climb, Strength, and Rock Smash to get to the top of Mt. Coronet (Spear Pillar).

Battle and defeat two Galactic Grunts when you get to Spear Pillar. You'll then see Cyrus, Jupiter, Saturn, and Mars all together. Cyrus will summon Dialga (or Palkia if you have have the Pearl version) and you will see some pretty interesting animation sequences for this.

Wait until Cyrus is done talking. Mars will challenge you to a battle. Your rival will suddenly arrive and battle Jupiter and it becomes a double battle. After defeating them, your rival will heal your Pokémon for you. You'll then see Dialga starting to form a new universe under Cyrus' control. The Legendary Trio arrives and destroys the Red Chain, which destroys Cyrus' control.

Know that Cyrus will not be pleased about this & battle you out of fury. Defeat him like you have done with all the other Galactic Team members. After defeating him, Team Galactic will disappear. Professor Rowan and Dawn (or Lucas if your character is a girl) will arrive and tell you to help Dialga who is now unable to control itself. You can do do this by capturing it (recommended) or defeating it.