Get All the Pokémon in Diamond and Pearl

If you want to get all the Pokémon in Diamond and Pearl then you want to read this.

Steps

Acquire an AR (Action Replay).
Insert the Pokémon Diamond or Pearl game and when it brings you to the AR homepage click get all Pokémon in the regular Pokédex.
Go to Professor Rowan in Sandgem town and talk to him.
Press A-B and select after he gives you the National Pokédex and he gives you the National Pokédex.
Go to your Pokédex and see if you have the National Pokédex and all the Pokémon.
Go to the GTS (Global Trade Station) and trade for some new Pokémon or put some up on offer because now you've seen all the Pokémon and you can trade for all of them.
- The GTS can get anyone in the world's Pokémon for trade, but it is best if you buy both games and trade with yourself.

The GTS needs WiFi, so be sure to have wireless Internet (broadband) connection or a WiFi USB.
Make sure you get the correct AR or else it might not work.

This counts as "cheating" in most gaming circles, so check with those you usually play with to see what they count as cheating and what they're okay with.
It may screw up your game if done wrong.