Instructions
Get the National Dex by seeing every type of Pokemon in the game. You will see most of the Pokemon by playing through and beating the game. Others are very rare and only found in one place. Professor Rowan gives you the National Dex after you see all 210 Pokemon.
Head to Stark Mountain after beating the game. Stark Mountain is located north of Route 227. Make your way through the caves in Stark Mountain. Buck, one of your friends, joins you partway through your journey through the caves.
Progress through the cave until you can go no further. Buck picks up a stone on the ground and tells you to meet him at his house. Exit the cave and go to the Survival Area located west of Route 225.
Go to Buck's house located on the left side of the Pokemon Center. Speak to Buck and he will tell you about Heatran, a legendary Pokemon living in Stark Mountain.
Return to Stark Mountain and go back to the room where Buck found the stone. Heatran is now there waiting for you. You can either knock him out or capture him.
Head to Celestic Town, located between Routes 210 and 211. Walk up to the mural on the northern wall and press the "A" button. Gym leader Cynthia appears to tell you about Dialga.
Go to the house located in the northern-most part of town. Talk to Cynthia's grandma inside the house to hear more about Dialga.
Go to Mount Coronet. Entrances are located on Routes 216, 211, 207 and 208. Head to the waterfall cavern and then enter the room next to the waterfall. Pick up the Adamant Orb and the Lustrous Orb.
Go to the Spear Pillar located on top of Mount Coronet. Look at the blue portal and Dialga appears to battle you. Capture Dialga with a Master Ball if you still have one or throw Ultra Balls at it until it is caught.