So you snapped up a 3DS before the price cut but never heard of this Ambassador Programme-whatsit thingy until just recently and, alas, you missed the eShop deadline. Don't worry, it's not too late.
There are clearly plenty of early 3DS adopters out there who never bothered logging into the eShop before August 12 to qualify as an ambassador, because Nintendo has updated its Ambassador Programme details page to announce other ways these negligent few can still qualify for the 20 free games promised.
To qualify, you need to have created a Friend Code and connected to the Internet with your Nintendo 3DS system prior to August 12th, or have kept your original receipt or proof of purchase for the 3DS.
With either of those, you will be able to register your 3DS for eligibility with an online tool NOE says it will launch "very soon". That tool is not yet live, but will be found here when it does launch.
Nintendo of America's Ambassador Programme Eligibility check tool has already gone live, so EU can't be far behind.