Thank you for contacting us about your Popup plugin issue.
Please clarify you expression more detailed –
However, the subscribe option is not visible on the site.
What did you mean? Inside your site WordPress admin area? Do you see the License tab? If no, please reactivate the plugin one more time – most likely plugin was installed incorrectly. And then use the instruction – https://supsystic.com/documentation/popup-mailchimp-integration/
Otherwise, perhaps you can provide more detailed description. We are always happy to help you.