If you're unable to view the Preview Mode for your campaign, your browser may be configured to block all pop-up windows. While this is a great way to prevent unwanted pop-ups while browsing the web, it can interfere with legitimate pop-up functions in applications like Mailchimp.
In this article, you'll find information about pop-ups for some popular browsers. But, because browser's are updated frequently, it maybe be best to view your browser's help pages for the most up-to-date information on how to enable pop-ups.
In Firefox, you can add specific websites to an allowed or exceptions list. Once you add a site to this list, pop-ups from that site are always visible. Mozilla's support pages show you how to manage pop-up blocker settings in Firefox.
Similar to Firefox, in Chrome you can add specific sites to an allowed list so pop-ups for that site always appear. For more information on how to manage pop-up settings in Chrome, have a look at Google's support documentation.
If you've enabled Safari's pop-up blocker, you can still view your campaign's Popup Preview as expected. Safari's pop-up blocker doesn't block pop-ups from opening if you've clicked on a button or link on a web page.
Microsoft's FAQ shows you how to turn Internet Explorer's pop-up blocker on and off. Choose your browser version from the drop-down menu on the upper right.