Instructions
Place "Platinum" into the Nintendo DS. Travel to Veilstone City and find the Pokemon Center. From there, enter the house to the northeast. Speak to the man in the house, and he will give you a Level 25 Porygon.
Insert "Diamond" or "Pearl" into the Nintendo DS if you do not have "Platinum." Turn on the system. Resume a saved game or start a new game. Travel along Route 212, where you will see a mansion. The mansion is owned by Mr. Backlot. Enter the mansion and speak with him. He will mention seeing a different Pokemon in his Trophy Garden each day. Wait until he mentions Porygon.
Enter the Trophy Garden in "Diamond" or "Pearl" and roam around. Players can expect to encounter Porygon in one out of every 20 random encounters in the Trophy Garden during the afternoon and night. If Mr. Backlot mentioned Porygon when you talked to him, players also have a five percent chance of finding Porygon in the morning. Porygon appears at Levels 16 and 18. Weaken it and use a Pokeball to capture it. Save your game.
Enlist the assistance of a friend. The friend should insert "SoulSilver" into a second Nintendo DS system. Both players should obtain the Pal Pad from the Oreburgh City Wi-Fi Club in order to trade with one another. The Pal Pad is found inside of the players' "Key Items" list. Select "Code" to view and exchange your code with your friend. Select "Register Friend Code" to enter the code your friend gave you. Both players must complete this process.
Travel to a Pokemon Center and speak to the Wi-Fi Club Representative. Answer "yes" to her questions four times to enter the trade screen. In the menu here, select your friend's name and click "Invite to Trade." Select Porygon to trade to "SoulSilver" while you are playing "Diamond," "Pearl" or "Platinum." Accept the trade. Porygon now appears in "SoulSilver" without any involvement from the Celadon City Game Corner.