Pokemon.com has announced a new Nintendo WiFi download event for Pokemon HeartGold and SoulSilver.
From July 31st through August 27th you can download the Enigma Stone for HeartGold and SoulSilver.
By default, the Pokemon Latias only appears in HeartGold while Latios only appears in SoulSilver. But the Enigma Stone sets off a chain of events in the game that leads to an encounter with the Pokemon that does not generally appear in game.
So if you have HeartGold you will encounter Latios, while SoulSilver players will encounter Latias.
To get the Enigma Stone, you have to have obtained the National Pokedex in HeartGold or SoulSilver, which means you have completed the Johto portion of the game and have defeated Lance and the Elite Four.
This is not an in-store download. You must connect to the Internet via your Nintendo DS using the WiFi option in order to obtain the Engima Stone. Instructions on how to configure your DS are in the instruction booklet that came with Pokemon HeartGold and SoulSilver. (page 35)
If you need further help, please see the Nintendo WiFi site. Do not post questions about Nintendo WiFi in the comments here.