Ensure CB Paid Subscriptions plugin in CB > Plugin Management is published and public. Now ensure Subscriptions tab in CB > Tab Management is published and public. Next navigate to CBSubs > Plans and ensure your plans are published and are accessible to the public under their Access tab. Now also review the Workflows tab of your plans to ensure they allow new registrations.
I don't know what more to suggest. Simply installing CB Paid Subscriptions, publishing it, and making a plan leaving all the defaults will show by default on registration. Only reason not to is a plan or CBSubs itself is not configured to display on registration. The alternative is you may not be using CB registration; please ensure you are using CBs registration page and not Joomlas or some other 3rd party registration.