You can't get them in game B2W2. You can only get them in BW and transfer them to B2W2.
In BW, you get a message from the lady at the gate telling you about the weather just before you enter Route 10 from Opelucid City. You can only get Thundurus in White, Tornadus in Black, and Landorus if you have both Thundurus and Tornadus in your party when you reach the Abundant Shrine.
The other way to get them in B2W2 is via Dream Radar, in which they will have their hidden abilities.