Register a Nintendo 3DS or Nintendo 3DS XL system and one of 15 games between 27th November and January 13th 2014.
You need to register Console and game codes after 27th November. Anything registered before doesnt count!
Specifically, registration needs to be from 3.01pm onwards on 27 November, so don't be caught out and register before then! If buying from the eShop or redeeming a code there, you also need to link your 3DS to your Club Nintendo account first, then wait "up to 24 hours" after linking the 3DS before buying/redeeming the title.