You can go to the White Forest, catch the rest of the Legendary Pokemon in the game, transfer Pokemon from Generation IV games over, and you also go around Unova arresting the Seven Sages.
You can go there after beating the elite four.
At 'N's Castle after beating the Elite Four.
When youre versing the champion
in what game of Pokemon
route 7 after beating the elite 4
You will receive it after beating the elite four and defeating Team Plasma.
FAP
you can transfer Pokemon from Pokemon platinum to black and white but you can't trade from black to platinum
You find him in meteor cave after beating the elite four.
Beat them and the champion
hacks. and pokemon that are able to evolve with the dusk stone won't until you get the new pokedex (after beating the elite four)
There is only one rod in Pokemon Black and White. You will receive it after beating the elite four. It is the SUPER ROD. it lets you catch a ton of new pokemon.