You can't make him appear in your game (Pokemon White) by itself - Tornadus is only availible in Pokemon Black. So, you will need to trade with someone who has a Tornadus. Once you have done that, get both Thundurus and Tornadus in your team and head for the Abundant Shrine present in Route 14. There you will have a chance to catch Landorus. If you fail, defeat the Pokemon League again, and he will return to the same spot.
Hope this helps... :3