Update: Fans have had problems when it comes to registering digital games on the Club Nintendo service, missing out on the Super Smash Bros. promo.
In order to claim a Mewtwo download and soundtrack, Club Nintendo members must register copies of Super Smash Bros. for Wii U and Super Smash Bros. for Nintendo 3DS. However, within the terms and conditions is a note that consoles must be linked prior to download or within 5 days of downloading the game. If not, the game will not be valid for the promotion.
An email from Nintendo support notes:
I am sorry to hear that you are experiencing an issue with registering your downloaded Super Smash Bros. for 3DS game.In order to register a game downloaded from the Nintendo eShop your console needs to have been linked to Club Nintendo either prior or within 5 days of downloading the game. If this is not done then the game will not be valid for the promotion.
Please note that this system does not work retroactively, this means if you have not registered the console with 5 days of the game purchase then you are not able to register the game. We are also unable to manually add any purchases made before linking the console.
Thanks to JayUK for the tip.
Have you registered your digital copy of Super Smash Bros. yet?