The most common reason this happens is if you completed your order as a “Guest”.
If you complete the checkout process as a “Guest”, then the serial number will not be added to your account automatically.
Once you complete the order at toontrack.com, you should get an order confirmation sent to the email address submitted on your order.
This order confirmation should contain the serial number you can use to register the product to your account.
You can register the serial number at toontrack.com > My Account > My Products area or in the Product Manager where it says “Register New Product”.