Instructions
Go to the southeast corner of Snowpoint City, and talk to the sailor standing next to the ship. The ship will take you to the Battle Zone. Your rival will go to the Battle Tower, so go there and challenge him to a fight.
Defeat your rival 1,000 times to unlock the Hoenn region. Your rival will be weaker than you, but choose strong Pokemon for your team to win the fights more quickly, because you'll need to win a lot of them to unlock all three extra regions.
Defeat your rival 1,500 times to unlock the Johto region.
Defeat your rival 2,000 times to unlock the Kanto region.