By default, the submit button from the Mailchimp html opt-in form will not be visible in the editor on preview mode.
Note: It will be visible on your live page, but not on the preview mode in the editor.
The reason for this behavior is in the default Mailchimp opt-in (sign-up) form generator which hides the submit button, but it can be fixed very easily.
All you need to do is to replace the class="clear" value from the submit button in the html code from "clear" to something else (or completely remove the class).
To find this class you can do that by either search through the html code manually and look for the button in the html code and remove the class.
Or you can just search for that class by clicking on "Control+F" and type class="clear" in the search bar which will directly take you there.
The submit button has "type="submit", you can use this for recognition.
After the class value is changed or removed from the html code, the subscribe button will appear on the preview mode in the funnelvio editor.