The code (at least for PC Digital download) has been emailed to you. Make sure to check your Spam folder as it is where it was stored for me, and apparently, many others. And no, you don't need ARR CE to get your 30 days free (i got myself the standard edition). When you go on Mogstation, make sure not to click on "Add new service account". Rather, click on the icon located on the bottom left of the page (the icon is the same used for Story quests). Input your registration code, and voilà!