Instructions
Find someone who will trade a Celebi to you. Make sure the other player originally downloaded the Celebi to "Pokemon Diamond," "Pearl" or "Platinum." If he downloaded it to another "HeartGold" or "SoulSilver" game, it will not work in your "HeartGold" game.
Start "HeartGold" and go to a Pokemon Center in any major town.
Go upstairs and enter the Union room if you plan to trade with your friend using the wireless ports on the DS systems. Your friend must also enter the Union Room, and you both must have the Pokemon you plan to trade in your parties. When you see your friend's character, initiate a conversation with him and follow the on-screen instructions to complete the trade.
Go downstairs to the Wi-Fi Club if you plan to make the trade over the Internet. You and your friend must have registered each other's "friend codes" in the game before trading can take place. Once downstairs, talk to either of the women at the center counter. The next screen will show which of your registered friends are currently online. Choose the friend who you plan to trade with, and follow the on-screen instructions to complete the trade.
Go to the Global Trade Center in the basement of the Pokemon Center if you do not have a friend willing to trade a Celebi to you. Go to the "GTS" counter and speak to the woman. Select the "trade" option and follow the instructions. The system allows you to put one of your Pokemon up for trade and ask for a Celebi in return. You leave your Pokemon there and return periodically to see if someone has accepted the trade.
Put your newly acquired Celebi in the first slot of your party and visit the Ilex Forest Shrine, which is to the west of Azalea town. This will trigger the special event that takes you to the past.