As mentioned before, exclusive download codes for the Pokémon Omega Ruby and Alpha Sapphire Special Demo Version have been distributed via e-mail to all North American members of the Pokémon Trainer Club on Pokemon.com that have opted-in to receive e-mail and were registered prior to October 20th.
The above video shows the main portion of the Special Demo Version in English, which you can watch if you haven't gotten your code yet, didn't register in time, or just want to hear Marriland's reaction to the demo.
Japanese players of the Special Demo Version received a Steelix with the Steelixite during their playthrough, while those outside of Japan gained access to a Glalie with the Glalilite. These stones allow evolution into Mega Steelix and Mega Glalie, plus the respective Pokémon and its Mega Stone can be sent to the full version of either game once it is available if you unlock it on the demo.
If you're registered for the Pokémon Trainer Club and are in North America, but haven't yet received your code via e-mail or are having difficulty locating your code, this official support post should hopefully help clear things up. If you live outside of North America, you can find out where to get your demo code on the Pokémon Omega Ruby & Alpha Sapphire Demo information page.