From the site:
Nintendo 3DS Ambassador Eligibility
To verify that you are eligible for the Nintendo 3DS Ambassador program, enter the serial number in the box below. The serial number for a Nintendo 3DS is located on the bottom of the system - just below the barcode - and starts with the letters CW followed by 9 digits. (Be sure to include the last number in the box.)
I checked, and mine seemed to be alright. I got this message:
"CW000000000" <==== (not my actual serial number, just used as an example)
You are a Nintendo 3DS Ambassador!
Starting Sept. 1, Ambassadors will be able to download 10 NES™ Virtual Console™ games at no charge from the Nintendo eShop. By the end of 2011, Nintendo will also provide Ambassadors with 10 Game Boy™ Advance Virtual Console games. Look for more information soon.
Important: If you have not already done so, connect to the Nintendo eShop today to make sure you are ready to download your free games starting Sept. 1. (How To) Connecting to the Nintendo eShop for the first time after Sept. 1 could delay the availability of the games.
So now it's just waiting 10 days or so....