The Bold Subscriptions "Manage Subscription" page is linked to Shopify customer accounts. Your customers log into their account in your store and have a "Manage Subscription" link available to them. When they select this link they are directed to the Subscriptions customer portal as expected. If you don't have customer accounts enabled in Shopify, this will not happen.
Visit Shopify Account Settings for Bold Subscriptions for detailed instructions to enable this feature.
With this change, customers can register for their accounts as well as login and manage their subscription.