You have to catch Reshiram or Zekrom [depending on the version] beat N and Ghetsis and it will be open, but to do this you have to beat the Elite Four.
Before Black City and after Marvelous Bridge.
Take a gengar to her and you'll find out
It can be found on Driftveil Drawbridge or Marvelous Bridge.
At the marvelous bridge on the right side
Help the little girl and defeat commander mars.
Beat the Pokemon League
There is a guy who will sell you one on marvelous bridge.
Before Black City and after Marvelous Bridge.
Take a gengar to her and you'll find out
It's a lady who is a girl that is weird
Go to the marvelous bridge and you can catch one
It can be found on Driftveil Drawbridge or Marvelous Bridge.
At the marvelous bridge on the right side
Help the little girl and defeat commander mars.
on marvelous bridge a guy will sell you a magikarp for 500 hope i helped
Annihilate the Elite 4 twice, go to Nimbasa, head up and then right to Route 16. Get to the end of it and enter Marvelous Bridge.
A merchant sells it to you on the Marvelous Bridge for 500 coins,dollars,or whatever you call it.