You CANNOT catch any of the Pokémon that are not in the Sinnoh Pokédex until you acquire the National Pokédex. You must SEE all 150 Sinnoh Pokémon. Once you do so, you must visit Prof. Rowan in Sandgem Town. When you talk to him, Prof. Oak will rush in and upgrade your Pokédex to the NationalDex. Once he does so, you can import any Pokémon from FireRed, LeafGreen, Ruby, Sapphire, and Emerald, by putting that game in the DS's GBA slot and selecting the TRANSFER option at the Diamond/Pearl Main Menu. Once you transfer Pokémon from the GBA games, they will be gone forever from the GBA games. After you tranfer Pokémon to Diamond/Pearl, you must then capture them in the Pal Park**.
- (To see Dialga [in Pearl]/Palkia [in Diamond] you must defeat the Elite Four and speak with the Elder of Celestic Town [The old lady in the big house.] She will then show you a picture of Dialga/Palkia, adding the 150th Pokémon to the Sinnoh Pokédex.)
**The Pal Park is directly south of Sandgem Town. Go to Sandgem Beach and Surf to the area near the Pal Park. (NOTE: I must confirm the Route #, though I believe they are Route 211[Water] and Route 212[Land])