The Jirachi is currently only available for download in Japanese games...
But anyway, to access the Mystery Gift, you have to:
- at least beat the 1st gym, so you can get inside the TV building
- go to the 3rd floor, speak to the man standing beside a woman
- tell him "Everyone" and "Happy", then "Wi-Fi" and "Connection"
- now you'll be able to access the Mystery Gift section from the main menu (the menu when you choose new game, directly after the glowing Giratina part)
- Set your internet connection beforehand
- Go to the Mystery Gift section, choose Receive Gift, and Download via WFC/Wi-Fi (IIRC we're having the rotom key / secret key event going)
Phew guess that's all ..