You need to catch N's Reshiram at Dragonspiral Tower first. To do so, head to Victory Road and maneuver until you see a Zoroark. The Zoroark will lead you to N'S Castle. Then, go with N to the top floor and battle his Reshiram. After defeating him, he will convert his Reshiram to a Light Stone and give it to you. Take it to Dragonspiral Tower and go to the top floor. Reshiram will then appear from the stone. Catch it. N will come by and say that Kyurem has returned.