Oh, that's easy. I just did that a day ago on the same version. You have to go into the underground tunnel and turn to the first (or second. I don't remember) and a team rocket guy will be standing there. Talk to him. He will give you a team rocket uniform. Then you can get in the radio tower and beat all the loser grunts and executives. After you get as far as you can in the radio tower ( because it won't let you go any further due to a big door) you have to go under the big mall and rescue the owner of the radio tower. Then he'll give you the card key and you can finish of the rest of the pathetic executives. Once you get to the top, you battle the top executive that is second in command under Giovanni. When you beat him, he will disband team rocket and then they're gone. Once you beat those queers, that weird guy in Mahogany Town will let you pass through route 44 to Blackthorn City. Then it just gets worse. You have to navigate your way through the ice path. That sucks. I'm still struggling in there...
