It is possible that the subscription was purchased using the 'Guest Checkout', which means a customer account will have not been created.
To manage your subscription, you can create an account using the same email address that was used to purchase the subscription. Navigate to the login page here and select 'New Customer? Sign Up'.
Once you have completed this click on the 'Manage Subscription' link on the left hand side of your account page.
If you have any problems please contact us.