You must beat the Elite Four to pass the policeman. To do this, you must have captured Lugia (SS) or Ho-oh(HG) and beaten the eight gym leaders. After that, with your new HM, Waterfall, Surf on the water left of New Bark Town and enter Tohjo Falls. Once past, go in the house and collect TM Sandstorm from the lady, and then go into Route 27 and 28. Once you are at the top, the police will check you for all your badges. After he lets you enter the Pokemon League, beat the Elite Four and the Champion, Lance. After this, you can go into the Kanto region.
After beating Kanto's last gym leader, Blue, go back to the area of the Entreance Of The Pokemon League and go past the Police man on the left - which now works. Reach the summit on Mt. Silver and Fight Red!