BLACK 1- To make Kyurem appear, go down the stairs, then northwest, and drop down the ledge. Then go north and along the north wall, but skip the first ledge that you reach, and continue east to find another ledge. Drop down it. You will see some stairs, but you can't reach them right now, so just ignore them.
Go southeast and drop down another ledge. Go south and then west. You will pass by TM03 Psyshock. Don't drop down the ledge that is southwest of the Psyshock or you will have to start over from the beginning of the area. Instead, head northwest and drop down a ledge.
From there, go right (through a bunch of tall grass) and down. You will find a small pond. Walk up to it and you will hear a pokémon sound and it will start to snow. After that, the whole area is covered in snow. Go northeast to find some stairs.
BLACK 2- (Black Kyurem/White Kyurem cannot be captured because Ghetsis's "staff" when you first encounter it but when you recieve N's Zekrom (Black 2)/Reshiram (White 2), you can come back to the Giant Chasm (do the same as in black 1)and battle it again. When you come back, it will be at Lv. 70 in it's normal form and to become Black Kyurem/White Kyurem, it will need to be combined with Zekrom/Reshiram by using an item called the DNA Link Pin.