Try going to your Cakewalk Store Account, and if the transaction completed, I believe you should see the download info, as well as your serial number, there.
IF that does not work, then send a Private Message (PM) to one of the Cakewalk staff - perhaps Ryan Munnis, in which you paste in your order information and payment confirmation, and I would imagine he will either straighten it out or will have someone else there straighten it out.
Welcome to the X3 family - they will get it fixed up for you.
You can look near the top of this forum, for the permanent thread titled How to Contact Cakewalk Directly, and you will see a link for Ryan Munnis as the last poster for that thread. Hover your mouse over his name/link, and you will see an option to send him a Private Message (PM).
I hope that helps,
Bob Bone