You have to beat Team Rocket at the radio station. It confused me too. Talk to the one guy guarding the stairs in the radio station and he will tell you to get a uniform. You can get one in the underground tunnel thing where they take pictures. You can take it from there.
You have to beat the gym in mahogany town then go to goldenrod city and finish the radio station then he lets you past ok :)
Radio tower beat team rocket