To install an extension, first of all, you should have already installed Popup Builder Free Plugin (please follow the link, in case you don't have it yet)
Now, let's see how you can install your needed extension, step by step.
First, browse Official Website - https://popup-builder.com/ and choose the section Extensions to find your desired one
After purchasing, please press Account and follow the link to save the file on your computer, as it is captured on the screenshot below
Second, please open your WordPress Dashboard >> Plugins >> Add New >> Upload Plugin, choose "downloaded.zip.file" from your downloads, after then, please click on the button Install Now.
Third, press Activate
The last step, please choose the button Popup Builder from the menu bar, press Add New to find the appropriate extension. After, the extension will be ready to use
Please consider, there are 2 types of extensions Type and Event. Types are visible icons on your Popup Builder menu bar. Event extensions are not visible on the icon menu bar, they are inside each extension.