Zekrom/Reshiram & Kyurem

First fly to the Elite Four, go through the old Victory Road's cave.. After that, find an entrance to the cave that has Zoroark outside (There's actually 4 entrances to the cave here, this cave is the 3rd from left).. When you get close enough, it will lead you inside the cave..

 
Just follow him and you will eventually enter the old castle and meet N.. Here N will follow you, for a few.. Seconds.. Enter the first room, N will talk about his past.. Then go out from the room and to meet N again.. Walk to the right side, there's a hidden Overheat TM there too.. After you have taken that, walk all the way to the top.. And finally you will enter the room where in BW you defeated N.. Here, N will challenge you with lvl 70 Zekrom/Reshiram.. No, it's for sale, you can't catch it yet as it still belongs to N..  After you have won, you will received the Dark/White Orb from him..

Go/fly to Iccirus City (ex-7th gym), then head to DragonSpiral Tower(at north).. Go through the tower and at the top you will be asked to use the Orb here.. Use it and you will battle again with Lvl 70 Zekrom/Reshiram..

After you have caught it, go to Giant Chasm.. Go to the place where you fought Black/White Kyurem for the first time.. Now, you will see Kyurem (In case you don't know, you won't see Kyurem before you battle Zekrom/Reshiram in the tower).. Well, of course capture it and you will get the DNA Splicer which able to combine both Zekrom/Reshiram and Kyurem.

How to Combine:
Have both Zekrom/Reshiram and Kyurem in your party..Go to key item, and use that stick.. Select Kyurem, then Zekrom/Reshiram.. You will combine both Pokes into one.. Just use the item on the Black/White Kyurem if you want to reverse the process.