From Bulbapedia, the community-driven Pokémon encyclopedia.
Pal Park (Japanese: パルパーク Pal Park) is a location on Route 221 in Sinnoh.
To unlock Pal Park, the player needs to have seen all Pokémon in Sinnoh's regional Pokédex and then show their accomplishment to Professor Rowan in his lab. After this has been done, Professor Oak himself will show up to both upgrade the Pokédex to National mode and to invite the player to come to Pal Park.
Pal Park allows people that have both the Generation III and Generation IV games to transfer Pokémon ahead via the Nintendo DS's Game Boy Advance slot. However, only six Pokémon per day can be transferred per GBA cartridge, and no Pokémon can be transferred until the previous six Pokémon have been caught. Using this, as opposed to other methods of getting previous generations' Pokémon, allows not only that other regions' starter Pokémon can be used by players of Diamond and Pearl, but also provides a way for players to take the Pokémon that they had raised in Generation III to the next level.
Unlike in Generation II, where the Time Capsule allowed Pokémon to return to Generation I, the transfer from Generation III to Generation IV is permanent, and all Pokémon sent over are removed from the GBA games.
Like the Time Capsule, though, there is a move requirement on the Pokémon to be transfered. This is, of course, that the Pokémon moving from Generation III to Generation IV know no HM moves.
After Pokémon have been imported from the GBA game in the bottom slot of the DS through the main menu of the Diamond or Pearl game, they may be captured in Pal Park through an event known as a Catching Show. Each Pokémon has a specific area of the park that they reside in, and therefore, they must also be found before they can be received. Pal Park uses a special type of Poké Ball that is colored gold and silver, which has a 100% capture rate, much like the Master Ball.
After a Pokémon is captured, if its status screen is viewed, it will display the region that it was originally captured in (Hoenn for Ruby, Sapphire and Emerald; Kanto for FireRed and LeafGreen, even if it was captured in an area of the Sevii Islands; and "distant land" for Colosseum and XD). It will also display the level that it was transferred over at.
Everything will stay the same, such as OT, ID number, item held and/or PokéRus if the Pokémon has it.
This is currently the only way to get the starters and legendaries of Kanto, Johto and Hoenn into Diamond or Pearl.
The spots of Spinda will stay the same as well as whatever form of Deoxys was transferred. Deoxys forms can later be changed at each one of the meteorites in Veilstone City.
The Park Ranger always gives the player a Berry as a reward. How fast the captures are completed and what Pokémon are transferred over will alter the score, with rarer Pokémon increasing it and common ones decreasing it.
Walking around in the grass