If you head through Victory Road, you'll come across a cave guarded by a Zoroark. After you've beaten the league, head back to this cave, and Zoroark will lead you in. You eventually come across N's Castle, and will find N inside. He'll battle you with his dragon. When you beat him, the dragon will turn back into the stone, and he'll give it to you. Take it up to the Dragonspiral Tower, and you'll get the chance to catch it. After you've done this, you can head back to the Giant Chasm, and are able to catch normal Kyurem. Kyurem leaves behind an item which allows you to fuse N's dragon with Kyurem to form Black or White Kyurem as you please.
Go south on Route 20, just past Sangi Town, and surf across the water. You'll come to the Heart Cave, and see Uxie. It'll fly off, and you can then find the pixies across Unova. Uxie is outside the Nacrene Museum, Mesprit is on top of the Celestial Tower, and Azelf is on Route 23. You have to stand in a certain spot for them to appear.
For Cresselia, you have to go to the Stranger's House in Yamaji Town. As you go up and downstairs, furniture will shift about, changing the path you can travel. Keep doing this until you can get in the centre room upstairs. A ghost will lead you in, and you will get the Lunar Wing. Take this to a certain spot on Marvellous Bridge, and Cresselia will fly in, letting you challenge it.
You will find the Magma Stone on Route 18. Take it to Rebirth Mountain, the mountain you went through with Bianca earlier, and Heatran will appear.
If you go to the Dreamyard, either Latias or Latios will appear, depending on version. You have to chase it down to challenge it.
In Driftveil City, the northern cave leads to the Clay Path. Follow that and you'll reach the Underground Ruins. There's a puzzle there; complete it, and you'll be able to challenge Regirock. Once you've done this, you'll be given one of two keys; the Steel Key or the Iceberg Key. Activate this key in the Unova Key menu, and you'll be able to challenge either Registeel or Regice. You can trade the keys between versions, so if you know someone else who has the other version, you can access both. Once you've caught all three, put them in your party and take them to Twist Mountain. Go down to the basement, and you'll find Regigigas. With the other 3 Regis in the party, it'll battle you.