Instructions
Enter the Global Terminal building within Goldenrod City, which is marked on your in-game map. The building is the westernmost one in the city.
Walk all the way to the back and press "A" beside the woman behind the desk.
Select "Trade" and then select "Yes" to save your game. Your character will automatically enter the next room.
Select "Yes" to establish a Wi-Fi connection. Wait a few moments for the game to connect and open a new screen.
Select "Seek Pokemon," followed by "Pokemon."
Select "ABC," followed by "A" and "Arceus." Select "Search," and all the trainers who are looking to trade an Arceus will appear in the top screen.
Tap "Select," and pick a trainer with the "Left" and "Right" buttons on the directional pad.
Press "A" to initiate a trade.