Defeat the Elite Four, go to the champions room, go to N's castle, catch Zekrom/Reshiram, defeat N and Ghestis, then you will be on your home, after talking with your mom and Looker, fly to Nimbasa City, go to Route 16 and your allowed on the bridge.
You can only access it after you have beaten the game (defeated the pokemon league) as White Forest is a post game place.
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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
It is Cresseliayou meet Cresselia after the hall of fame in pokemon black+white 2 you need to retrieve the lunar wing first from the Strange House near Reversal Mountain. then go to marvelous bridge and you will meet Cresselia
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.
It's a lady who is a girl that is weird
A merchant sells it to you on the Marvelous Bridge for 500 coins,dollars,or whatever you call it.
It relates to the ghost in pokemon white 2/black 2 where she haunts the strange house and in it is the lunar wing and then cresselia will appear on Marvelous bridge if you have the lunar wing.
The PokeTransfer machine is in the PokeTransfer Lab on Route 15.route 15 is near black city (pokemon black) or white forest (pokemon white) and Marvelous Bridge
Beat the Pokemon League
There is a guy who will sell you one on marvelous bridge.
I dont think you can even though you get the adament orb,the lustrous orb,and the griseous orb from the shadow triad on marvelous bridge.
You have to finish the game( beat the elite 4 and N ) and come back
The Marvelous Bridge is between Routes 15 and 16. East of Nimbasa City and west of Black City/White Forest.