If you do not have a Steam account, you will need to download the client from the official website, install it and then create your account. Once you are logged into your account, click on "Add game" in the bottom left corner and then on "Activate product on Steam". After a few clicks on "Next", you can finally enter your CD key, the game will automatically be added to your library and the download will begin.
This is Nintendo's download platform. After buying a Nintendo 3DS game, you will receive a serial. Start the Nintendo eShop from the Home menu, select Settings/Other, and then tap "Redeem Download Code" to enter your 16-digits code which will allow you to download your game.
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