The Nintendo 3DS Ambassador program is scheduled to kick off tomorrow, however it appears that's been initiated just a tad bit early. Eligible users can grab those 10 NES games for free right now!
It's all found in your downloads history under settings. In addition to the 10 free NES games, there's a little app called 'Ambassador Certificate' and it'll notify you when the next 10 games will be available for download.
At the moment, it appears that the games are available only in the US eShop, UK Ambassadors will have to wait until Nintendo officially releases them.