You need to actually get the Light Stone.
This can be done by heading to Victory Road. When doing so, a path that a Zoroark was blocking beforehand is now accessible. You'll end up in N's Castle. After meeting N and going to the top of his castle, you'll battle his Reshiram. After defeating N, his Reshiram will turn into a Light Stone and will be given to you. Then, head to DragonSpiral Tower with the Light Stone. Reshiram will appear. Catch it and head to Giant Chasm, where you fought White Kyurem before. Kyurem will be there if you caught Reshiram.