Pokémon Diamond and Pearl/Dual-slot mode

Dual-slot mode, often called the dongle method, is a way that you can use to obtain Pokémon that are out of the Sinnoh Dex, by using the GBA games of the series as dongles for the DS games.

With one of the five Game Boy Advance games inserted into the GBA slot of a Nintendo DS while playing either Diamond or Pearl, you will find wild Pokémon not otherwise available in Sinnoh (except direct migration at Pal Park).