Kaz, I hope you can follow this:
If you are registered with SP, go to your account, then to the top of the page where it says "do you need more credits".
Once you click on that you will go to a page where you have the option to purchase more credits, however if you go a little further down the page, there is a section which asks you if you have credits to redeem or some such thing.
Anyway, there is a box there where you enter the code that Ancestral Scotland emailed to you.
Good luck.