All work with the Mailchimp lists and newsletters is mainly conducted on the Mailchimp side. The Mailchimp Integration with eCommerce Support add-on in X-Cart allows viewing Mailchimp audiences uploaded to the store Admin area automatically and checking customers' subscriptions. The audiences creation and management and the subscription lists management are done on the Mailchimp side.
After you have connected your store to your Mailchimp account and configured the add-on Mailchimp Integration with eCommerce Support, Mailchimp audiences are automatically uploaded to the Marketing -> Mailchimp Lists section of your X-Cart Admin area:
For X-Cart 5.3.x and earlier, Mailchimp audiences are displayed in the Sales channels -> Mailchimp lists section.
Be sure to check this page and do as follows:
Use the ON/OFF button to enable/disable a specific list and groups for your store. The active lists and groups will be visible on the subscription forms for your store users. Inactive lists/groups will not appear on such forms, and thus your store users will not be able to select them.
Use the checkbox in the "Subscribe by default" column to set the default subscription lists and groups for users who opted in to receive your store's email newsletter via a single check box option. Such users don't select specific lists/groups, e.g., by subscribing via the "Sign up for _My_Store_ news" box in the site footer or by enabling the "Subscribe to our news list and be in touch with our latest offers" checkout option).
All Mailchimp lists recorded on the Marketing -> Mailchimp Lists page of your store Admin area are created and managed on the Mailchimp dashboard outside your X-Cart-based store. For more info on Mailchimp lists (called "audience" there), see Mailchimp Knowledge Base.
Any lists and list groups configured in your Mailchimp account before your X-Cart store has been connected to Mailchimp are automatically imported into your store. However, if the Marketing -> Mailchimp Lists page of your store Admin area does not display existing Mailchimp lists correctly (e.g., after you edited the audiences/groups), use the "Update Mailchimp Lists" button on the add-on settings page to synchronize data between the systems. For more details, see Uploading Store Data to Mailchimp.
Anyway, you need to ensure that the audiences and groups you have configured in Mailchimp are visible on the Marketing > Mailchimp Lists page in your X-Cart store's Admin area.