The code is MATSUBUSA, and you need to enter it into your copy of Pokemon Sun or Pokemon Moon.
Pokemon Sun and Moon players can now use a special code to access five Mega Stones, all which can be used for several evolutions, including Sceptilite, Blazikenite, Swampertite, Banettite, and Cameruptite.
After entering the code, you’ll get your Mega Stone Gift from the deliveryman at any Pokemon Center.