In order to access the Nature Preserve, you must first register Unova Pokedex #001-297 as seen in your Unova Pokedex. You do not need to actually catch all of the Pokemon, but you do need to at least see them.
Almost all of them can be seen within a single version, although there are three Pokemon that you may need to trade for in order to see, as no Trainer has them in the game:
- Rufflet (Black 2) or Vullaby (White 2)
- Snivy, Tepig, or Oshawott (whichever one you and Hugh didn't start with)
- Serperior, Emboar, or Samurott (whichever one you and Hugh didn't start with)
You must be able to trade in order to see those Pokemon.
Victini, Keldeo, Meloetta, and Genesect do not need to be seen in order to qualify.
Once you've seen those 297 Pokemon, head to Nuvema Town and talk with Professor Juniper inside of her lab. She'll reward you with the Permit, which you can take to the Mistralton Airport and fly to a special location: the Nature Preserve!
After catching all 297 Pokemon, instead of just seeing them, you can return to Professor Juniper and she will reward you with another special Key Item: the Round Charm. This item, while in your bag, increases the rate that eggs are produced in the Day Care, meaning it will speed up your breeding time substantially.
While it is certainly no easy feat catching all 297 Pokemon to complete the Unova Pokedex, the reward is definitely a pretty cool one, especially since it gives a constant effect.
In all of the previous Pokemon games, from Red and Blue all the way up to Black and White, completing your Pokedex was a major challenge. Your reward for accomplishing such a grueling feat? You got a diploma. A diploma. And a Trainer Star or improved color for your Trainer Card in later games.
In Black 2 and White 2, in addition to all of that, you also get a very special reward if you talk to Cedric Juniper with a complete National Dex: the Shiny Charm!
The Shiny Charm is similar to the Round Charm in the sense that it provides a constant effect once you've acquired it. It increases the chances that you'll encounter Shiny Pokemon, raising their encounter rate by an astonishing 3x, meaning you'll be three times as likely to run into a Shiny Pokemon than without it. This is a constant effect, too, so you'll always have that going on while training or fighting Pokemon in the wild. It even works while breeding, too. Keep in mind that they will still be quite rare, but it is definitely a valuable item and a great reward for completing your Pokedex.
Keep in mind that the following Pokemon don't need to be caught for the Pokedex to be counted as complete: Mew, Celebi, Jirachi, Deoxys, Phione, Manaphy, Darkrai, Shaymin, Arceus, Victini, Keldeo, Meloetta, and Genesect. These are all event-only Pokemon and were available only for a limited amount of time, whereas all of the others can be acquired naturally through some repeatable means.
As you can see, there are a variety of Pokemon that can be caught here, including rarer Pokemon that can't be caught elsewhere. They're also at pretty high levels, too (Magikarp and Gyarados can both reach level 100 in the wild!).
In the middle of the preserve, though, is your real prize: a Shiny Haxorus!
Definitely make sure you catch this rare Pokemon, because Shiny Pokemon are generally rare to find on their own, and one as cool as Haxorus is tough to pass on.