To get Kyurem you must first catch Zekrom or Reshiram from Dragonspiral Tower. It can be captured and used as regular Kyurem, or it can be merged with Zekrom or Reshiram to make Black Kyurem or White Kyurem. Kyurem is the main Legendary Pokemon of Black 2 and White 2.
Note: There are two Master Balls in Black 2 and White 2. If it's night, or you're in a cave, using a Dusk Ball will increase your chances a ton. If the battle is going on for some time, which it probably will, you can always use a Timer Ball to help out. Ultra Balls may be an obvious choice, but those things still may make it hard on you. There are many Poke Balls out there that can help you catch a Legendary Pokemon. You may accidentally make the Pokemon faint or you may get knocked out by going easy on the guy. If you really want to catch a Legendary Pokemon, you should save your game before attempting it. You can walk up to them and interact with them to initiate the battle.
Most Legendary Pokemon are stationary on the map.
We'll try to prevent you from smashing your (3)DS with a few tips on catching these Legendary Beasts. Legendary Pokemon are among the hardest Pokemon to capture in any game.