If you missed the opportunity to connect to Nintendo's eShop on your 3DS system, before August 12th 2011 there's no need to worry! Nintendo have now extended the rules for their Nintendo 3DS Ambassador Programme.
The service was originally only going to be available to those who connected to the eShop on their systems, but now their are more options that will get you your 20 free NES and GBA games.
Nintendo states that if you've connected your system to the internet and created a friend code before August 12th 2011, then you're still eligible to get the free games. Alternatively, you can use the receipt from when you bought your Nintendo 3DS to prove the date of your purchase. Nintendo will soon be putting up a tool which will allow you to enter data from your receipt. The same tool will also allow you to enter your Nintendo 3DS serial number, found on the bottom of our system. By entering it, you can confirm whether your system is eligible to receive the free games.
The tool will be available to access at later date from Nintendo 3DS Ambassador Programme.