I opened it when I finished Proffesor Elm's job in the beggining
Chat with our AI personalities
It is upstairs of the Pokemon center the first person to your left
you can you probably got a glitched game ..
go to the union room and trade with a different trainer (make sure someone else playing pearl, diamond, silver gold or any Pokemon game is near you and is in the union room
You find it in Union Cave.
walk?? look around!!