When using the Safari browser the PopupAlly plugin zip file may automatically unzip when downloaded. You’ll see an error message when trying to install the plugin if the file is not a .ZIP file.
You can disable the auto-unzip in the Safari preferences:
Safari -> Preferences -> General, then uncheck the “Open safe files” option.
Once you’ve changed this setting, download PopupAlly again, and you’ll have the .ZIP file needed to install the plugin on your site.