They put the game automatically on your Wii U if you've allowed the feature for that.
NintendObserver’s select content on Mini Mario & Friends: amiibo Challenge for Nintendo 3DS and Wii U.
Girders, movable pipes, long springs, boost pads, balloons, egg launchers, candles, fire blocks, bouncy mushrooms...
Nintendo of Europe's press release for the amiibo free-to-start title.
Each amiibo character has their own given ability.
Participating stores are GameStop, Best Buy and Amazon.com.
The free game is available through retail on March 25 and on the eShop from April 28. You'll just need a compatible amiibo.
Mario vs. Donkey Kong X amiibo.