It's likely that most of you have already connected your 3DS to the eShop at least once, but since the eShop is a big waste of time, I wouldn't be surprised by a few stragglers. If you bought a 3DS already and you want twenty old games for free, make sure you get yourself authenticated, as the deadline ends at midnight.
In order to be recognized as a "3DS Ambassador," you simply have to connect to the eShop via your 3DS. Once done, you'll have proven your gullibility to Nintendo, and thus be qualified for your freebies. Ten NES games and ten GBA games are at stake, so don't forget!
Also, remember that Walmart is selling discounted systems early, so you could get a 3DS for cheap and steal from extra benefits. Beautiful.