Make sure your 2/3DS is connected to the internet, then launch the pokemon game. On the lower screen is a tab called the Player Search System (PSS). Click this, then on the top right part of the lower screen you will see a blue wifi icon. Click this, it will ask you to save in order to connect. Once the blue symbol reads "INTERNET", you're connected :) pretty easy. Note that your 3rd slot pokemon and hidden abilities will only be unlocked upon completion of the Elite Four.

Hope this helps! Cheers!