Question

Where is N's castle in Black and where do I catch Reshiram?

Answer 1

When you beat the Elite 4 for the first time, you will go face the champion as usual. But before you get the chance, N's Castle will rise from behind the Pokemon League.

You will go inside with your Light Stone and it will react when N's Zekrom comes out. Reshiram will appear and you catch it then.

Answer 2

N's castle is a one time visit in B/W you beat the E4 and you battle N in his castle.
You can catch Reshiram in his castle when it comes out of the Light Stone.If you faint it you can catch it at Dragonspiral Tower.

Answer 3

You find reshiram at the very end of his castle right before you battle N on the center room of the top floor after you beat the elite 4