Get into it? Or reach Kyurem once you find it? I'll answer both. Giant Chasm is north of route 13, right next to Lacunosa town, so you have to go right from the pokecenter and south through the grass...
you must go around the chasm and go deep into the middle. in the middle there will be a path somewhere by the grass.you must go down the path and there willl be the little pond.a blizzard will...
go up from the then go to the lower right corner keep along the northern wall and you will come to a new area imetitley keep the right wall there is the pond you will hear a Pokemon when you walk up...