Instructions
Insert a Game Boy Advance cartridge containing a saved game from "Pokemon FireRed" or "Pokemon LeafGreen." You will only be able to transfer a Venusaur to your "Pokemon Diamond" game if you have already captured one in an earlier version. If you do not have a Venusaur in a previous game, offer to trade a friend another Pokemon for the Venusaur.
Insert the Game Boy Advance cartridge containing your saved game into the GBA slot in your Nintendo DS. Do not remove your "Pokemon Diamond" cartridge. Turn on your DS and select "Pokemon Diamond" from the main menu. Use your stylus to select "Migrate Pokemon" from the title screen. This option will bring up a list of available Pokemon from your LeafGreen or FireRed game. Select up to six Pokemon, including the Venusaur, to transfer to the Pal Park. When the transfer is complete, you will be automatically returned to the main menu.
Press "Continue" to load your saved "Pokemon Diamond" game. Travel to the Pal Park, located at the east end of Route 221, directly south of Sandgem Town, and enter through the front gates. When you enter the Pal Park, you will receive six Park Balls, special pokeballs with a 100-percent catch rate. Walk around the park until you encounter the Venusaur. There are only two possible actions in park encounters: throw and run. Select "Throw" to capture the Venusaur in a Park Ball. When you capture the Venusaur, it will automatically join your party. If your current party is full, the Venusaur will be placed in storage. You can visit any Pokemon Center, located in most major towns and cities, to withdraw your Venusaur.