You can access your customer portal through your store account login. If you have not registered an account, you will need to create an account using the same email address used at checkout.
Once your account is registered, you can log in and access your customer portal by clicking on the Manage Subscriptions link.
You can also access your account using a direct link to the customer portal. This link is normally sent in the Subscription Confirmation email when you begin a subscription and the Upcoming Order email.