Just register any 3DS system and one of these 6 games between March 1 and March 31:
- Mario Kart 7
- Super Mario 3D Land
- Animal Crossing: New Leaf
- Donkey Kong Country Returns 3D
- LEGO City Undercover: The Chase Begins
- Yoshi's New Island
Only for US/Canada folks. More details on the link up there.