Landorus is a special pokemon that you will only encounter once in Pokemon Black.
After you get the National Pokedex, and once you have both Tornadus and Thundurus in your party (with one of them being caught in your game), head to the Abundant Shrine on Route 14. Click on the shrine, and as long as you have Tornadus and Thundurus in your party, Landorus will fly down and engage you.
You will need to trade for Thundurus, since you can only catch it in Pokemon White.
Landorus is level 70 when you encounter it and has a low catch rate, so come prepared with high level pokemon and a lot of pokeballs.