ITs kinda a process though.Still IGN posted the steps.
http://ds.ign.com/articles/119/1191768p1.html
Edit: you know what its kinda simple actually.
1. Go here http://www.nintendo.com/consumer/systems/3ds/en_na/3dsambassador.jsp?menu=how&submenu=ctr-ht-amb-lookup&source=online to see if your eligable
2. now log on to the e-shop
3. go to settings./other and click on your downloads
4. find the ambassador gaems and click re-download.








