Instructions
Defeat all of the Pokemon Gyms and the Elite Four. Each time you defeat a Pokemon Gym, you are allowed to use a different Hidden Move (HM.) These HMs give you access to new parts of the Sinnoh region and to the Pokemon that inhabit these new areas.
Catch all of the Pokemon you can on your own within your version (Diamond). Each version of the Pokemon game has different Pokemon available in the wild, so trading with other versions is necessary to catch them all.
Use the PokeRadar, given to you by Professor Rowan after you've seen every Pokemon in the Sinnoh region, to make different Pokemon appear in an area where it normally would not. For example, on Route 230, Togepi doesn't normally appear, but if you use the PokeRadar, there is a chance it may.
Transfer Pokemon from Pokemon Ruby, Sapphire, Emerald, Fire Red, and Leaf Green game packs using the Pal Park, found on Route 221.
Trade Pokemon with friends using the Nintendo Wi-Fi Connection. This is the easiest way to obtain Pokemon from other versions without owning them yourself.The Pokemon Global Trading System (GTS) was designed to allow simple trades between strangers over Wi-Fi, without requiring a Friend Code. You can find the GTS Building in Jubilife City.
Arrange trades with trainers online. If you absolutely cannot find the few remaining Pokemon you need on the GTS, use a website such as Friendcodes.com that is dedicated to arranging wireless play between two Nintendo systems.