To do this you must have a Nintendo DS or Nintendo DS Lite, it will not work with any later DS versions as they cannot play gameboy games. This will also not work with any Gameboy version because they cannot play DS games.
>In Diamond, Pearl, and Platinum, players can travel to Pal Park as soon as they have obtained HM03 (Surf) and earned the Relic Badge (DP) or Fen Badge (Pt). Access will be denied, however, until all the Pokémon in the Sinnoh Pokédex have been seen and the National Pokédex has been obtained. Until this is achieved, the construction workers outside will mention that Pal Park will be opening soon. Once the National Pokédex has been obtained, Professor Oak will meet the player on Route 221 and invite the player inside the building. He will briefly explain how Pal Park works, as well. Players will also find that the "migrate from [game]" option will appear on the main menu screen when the game is loaded, provided a GBA game is in the Nintendo DS's second slot.
As said above, you must have all of the Pokemon in the Sinnoh Pokedex to access Pal Park
Step 1: Insert Pokemon Sapphire into the Gameboy game slot of the DS, insert Pokemon Pearl into the DS game slot.
Step 2: Start up your Gen 4 game. (In this case, Pokemon Pearl.)
Step 3: On the main menu select "migrate from game." (In this case, Pokemon Sapphire.)
Step 4: Select the Pokemon you want to transfer from Pokemon Sapphire to Pokemon Pearl. (Note that Pokemon with HMs cannot be transferred.)
>After Pokémon have been imported from the GBA game in the bottom slot of the DS through the main menu of the Generation IV 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 known as a Park Ball, which has a 100% capture rate, much like the Master Ball. Once a battle begins, there are only two commands that can be used - Throw Ball, or Run. Throwing a Park Ball at the Pokémon will capture it with a 100% capture rate; running away will escape from the battle.
You can only transfer 6 Pokemon per cartridge every 24 hours.
(If it is like B/W) You have to transfer 6 Pokemon at a time. (For example, you couldn't just transfer a single Blaziken, you would have to transfer five other Pokemon.)
Hope I helped!