Did you do this today? If you did, then you won't be able to get the OR/AS demo. You have to "opt in" to get a code by email before the 20th. If not, then go to the eShop, get the system update, and go to the "Download Codes" section in the upper left of the eShop menu. Download your demo, and have fun! By the way, as far as I know, us North Americans get Mega Glalie.
Source: Downloading the demo 30 minutes ago, and living in North America