I ordered the Ultimate Collection Seeker's Edition digital download because my original discs were too badly scratched after 9 years. I had not received the registration code but all I needed was to add Seekers.
Go to your SE account management system and go to "Select Services" and choose FFXI, then click on "Add service Account" it says registration code but I entered the Product code that was in the email and it worked. Seeker's is now activated without the registration code.
This only works if you already have content ID and bought the entire collection as I did.