After you go beat team rocket and return to Mohogany Town as speed freak said, exit town to the right of the pokecenter. The man with the Rage Candy Bars allows you to pass. Contine along the route until you get to ice pass. Be sure to bring Strength with you, you'll need it to get through. Inside, be sure to pick up the Waterfall TM (it's not hard to find and a man will tell you where it is. You know that you're almost out when you reach a Kimona girl. Push her off the ice and then slide off yourself. The exit to the pass is right there and leads directly into Blackthorn City.
