When can I transfer Pokemon on to Black?

Answer 1

Once you have the National Dex, you can transfer them via the DS Download Play from one DS to another. There's a place in B/W that allows you to do this.

You must beat the game first.

Answer 2

you must first beat the Elite 4 and that opens up your nationaldex and the extralink u can also use DS download in the main DS menu wich allows you to send them over 6 at a time without trading or losing Pokemon...but the catch is you can never trade them back to that original DS

Answer 3

You have to get the national dex and go to the place that allows you to transfer pokemon via DS download.