How to Catch Legendaries on Pokémon Diamond or Pearl

Catching Dialga

On your Pokémon Diamond game, visit your rival at Lake Acuity, then take the key/card thing from the Team Galactic guy standing by the satellite dish in Veilstone City, get through the HQ, release the Pokémon in the testing room, defeat Cyrus, and there will be Dialga.

If you have the Pearl version, you'll need to trade with a friend or use an Action Replay code to obtain Dialga.

Catching Shaymin

To catch Shaymin before completing National Dex, go to Victory Road and find an exit that leads to the right. Follow the path until you find Shaymin.

To catch Shaymin after you have completed the National Dex, show it to either of the Professors. Prof. Oak will give you a choice of three Pokémon to capture, Shaymin is one of them.

Catching Heatran

During your first visit to the island with the Fight, Survival, and Resort areas,you can go to by the ship at Snowpoint City. Then go up when you get off the boat. You can get to Stark Mountain from there.

See a guy called Buck, battle him, and then go straight north, until you see the cave.

Save the game first here.

Then go through the maze inside, and let Buck take the Magma Stone.

Go back to survival area and go in the house closest to route 225.

Talk to Buck and he will put the magma stone back. Go back to the place were you found the Magma Stone and Heatran will be there.

Hatching Eevees

Get Eevees from Bebe.

Evolve it into any one of the seven evolutions.

Catch a Ditto from route 212.

Take Ditto and your Eevees evolutions to the day care in Solaceon Town until you talk to the Man outside. Or, check your Pokepad Day Care app; if an egg is in between them, hatch it by walking around with it.

When it hatches, it will definitely be a Level 1 Eevee.