As a Tertulia PRO subscriber, you can activate your newsletter pop-up by navigating to Design > Signup Popup > check the box next to "Show signup popup to capture emails".
From there, you can select whether or not to include a reader magnet when the user signs up through the pop-up. You can also change the text, the colors, and the images that appear on the pop-up. Toward the bottom of the Signup Popup settings, you'll see a bar labeled Trigger, which determines at what moment the pop-up will appear once a visitor gets to your homepage. You can check the box next to "Shows after 5 seconds" or "Shows after scrolling down 25%" or you can check both! Be sure to click Save Changes for your edits to appear on the live site.