How can ya download it than?
At particular times Nintendo decides to release an event. This will either be an Event where you have to go to a location or either send it out via Wi-Fi connection. These events can be obtained as a "Mystery Gift". After you obtain your Mystery gift You'll also get a wondercard. This wondercard you can trade with other friends so they will have the event too. However you can only have a maximum of 3 Wondercards on your game and have to delete one of them to obtain another gift.
I give two examples of events nintendo brings out:
- In my Soul Silver game I obtained an item, with this item I am able to encounter Latias by going to the Pewter City Museum and talk with a certain person over there. Latias appeared outside. This while Latios is the only pokémon available in Soul Silver without any event
- You'll recieve a pokémon without needing to capture it. These pokémon can be picked up from the pokémart from an mailman standing inside giving you the pokémon. This way I got a Mew Lv 5 from an event as well as the three legendary hounds in shiny form to later transfer them to Pokémon Black/White to trigger an Zoroark event which only can be affected by the Shiny Version of one of the three Hounds.
What kind of events are available depends on your country, continent, and time. For example if you are European and you have the European version of Pokémon black you cannot recieve an Wi-Fi event going on in Japan. Another point out is that some events are also for certain pokémon games.