I'm playing Pokemon Black and there are two characters blocking my path to the Marvelous Bridge. They are saying, "..Marvelous Bridge is being checked". In terms of progression I have made it to the Pokemon League and my game is currently in Spring. My thanks to anyone who knows a fix.
You must beat the Pokemon League before you can make it onto the Marvelous Bridge, as it is a post game only area.
Source: I played the game :3
Marvellous Bridge will unlock once the Elite 4 has been defeated. You say you have made it to the Pokemon League, but have you defeated them? If not, then that's your problem. If you have, then I have no idea what is wrong.