Okay so Kyurem is in the Giant Chasm. To get to Kyurem's Chamber in the Ice Cave;
Slide on over to the northeast,and do so until you reach a point where you'll have to get a little clever. Once you hit the edge with a ice shard to your left, you can get around it by sliding right, down, left, down, right, and then up. That'll get you over to the stairs leading down.
From there, it's really easy to get over to the steps leading down into the normal part of the chasm's forest. Do so and then head north, until you find stairs leading into a cave.
After some cutscenes you'll need to battle Kyurem-W/B make it faint... then you'll need to give Ghestsis a beating. Have fun! xD