Hi, Ice Master.
You go to the basement of the Pokemon Center in Phenac City.
There, you are able to trade to Ruby/Sapphire/FireRed/LeafGreen.
Just make sure that you have your Gameboy Advance Sp connected to
the second port of the Nintendo Gamecube via a Gamecube Link Cable
before you talk to the lady there.
I'm just don't remember if you have to reach a certain point in the game
before you can use the trading machine there.
I, however, still need to buy the $20.00 Gamecube Link Cable so I can
connect to Colosseum with my Crystal and LeafGreen Gameboy Color
and Advance Games.