Did you read the requirements for being an ambassador?
http://www.nintendo.com/consumer/systems/3ds/en_na/3dsambassador.jsp(Italic font added by me for emphasis.)
"Nintendo 3DS owners who performed the latest
system update or who
visited the Nintendo eShop using a wireless broadband Internet connection
prior to 11:59 p.m. Eastern time on Aug. 11, 2011, were automatically registered for the program and will receive 20 free downloadable games before the end of the year."
If you only connected on August 15th, then I'm not sure that you qualify. Did Nintendo change the rules or give other ways to become an ambassador? Maybe customer support can help if you have proof of purchase showing you owned the system before the deadline?
Edit:Did see this link for Australia and know Europe has something similar, but don't know if North America is offering the same option. Again, best to contact customer support.
http://www.kotaku.com.au/2011/08/didnt-register-for-nintendos-ambassador-program-theres-still-hope/Final Edit:Based on this quote that you posted, I'd say you are probably "screwed".
I just got my 3Ds today thanks to the price drop. To be honest, the 3D doesn't do much for me. I always think of it a gimmick..although for games it does look cool....
As for viewing it..I'm fine with it in theater...But for the 3DS, it does make my eyes hurt a tad..
The window for being an ambassador was closed at that point. The deal is only for people who bought before the price drop, and it sounds like you missed the cut by a couple of days.