Normally, you can't. You have to trade on GTS or Poke Transfer from D/P/Pt. You could also cheat.
However... there is a way to receive old event Pokemon from the internet in B/W, including Shiny Dialga, Palkia, and Giratina, based upon this expired event that ran in mid-2013. Here are the instructions to do it (no mods required).
The reason they give the orbs to you anyway is because you can't transfer items with Poke Transfer, so in case you do get the Pokemon, you have the three orbs ready.