The only exception are the four Event Legend Pokémon (Victini, Meloetta, Keldeo, and Genesect), which do not need to be seen in order to unlock Haxorus.
Professor Juniper can be found in her lab in Nuvema Town, where the player in Pokémon Black and White started the game.
Having a Pokémon with Hypnosis/Sleep Powder or Thunder Wave/Stun Spore will make capturing Haxorus much easier. Haxorus is level 60. At this point in the game, you should have a team that could take care of it without any problem, but the real concern is accidentally knocking it out before you capture it.
Enter the main terminal building and talk to the person behind the counter. When you present the Permit, you will be automatically flown to the Nature Preserve.
Hit the Haxorus with a few powerful hits to get rid of the majority of its health. Be careful not to KO it! Once the Haxorus’s health is in the red, hit it with False Swipes until it is down to 1 HP. False Swipes will not knock Haxorus out, so you can use it without fear. Put Haxorus to Sleep or Paralyze it. This will make it much easier to capture.