The Newsletter Pop Up plugin can be viewed within the Customizer preview, but requires that you supply form code first. Without this, the plugin can’t show you the form, because there isn’t one!
To preview your form:
1. Paste the MailChimp code into the Subscription Form Code box. Hit Publish. Important: Doing this will publicize any other changes you have made in the Customizer! However, you will need to publish your form code if you want to preview your pop-up, as displayed on the previous page.
2. Exit the Customizer completely, then return to it by going to Appearance > Customize. Now that the form code has committed, you should be able to see your pop-up in the Customizer Preview.
4. If at any time you find that your changes are not reflected in the preview, simply save your progress, exit the Customizer and re-enter. Once back, head to EmPress Pop Up again. Now, upon clicking the tab, you should see a preview of what the pop-up looks like. Click on Styling Options to adjust the appearance of your pop-up. You can select an Overlay Color (the flood of opaque color laid over all your site content, but behind the popup), the pop-up’s background color, an optional pop-up background image, and the text, input, and button colors for the pop-up.