You access it by going online on the Wii shopping channel. You gotta put nintendo points onto your console by either using a paid for points card or a credit card. You then find a game you wanna download, download it, and you can then access it from you Wii homescreen. If you download a game, you Wii will remember it and if for any reason you need to download it again it's free of charge. I don't think there much more to it than that. It's generally a lot simpler than the Ps3 and 360 market place so hopefully you shouldn't find it too confusing.
Why do you ask?
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I wandered though the weird and lurid landscape of another planet.