After you purchase EasyIndex Plus, you'll be redirected to a confirmation page that displays a download link and your license key. An email copy of the page is also sent to you.
Download the plugin by clicking the download link.
The file you download is a .zip file that can be installed via the standard WordPress "Add plugin" process but some browsers (notably Safari on some Macs) will, by default, automatically unzip the file after it's downloaded. If that's the case, you'll either have to turn off the auto unzip option in the browser settings and re-download the file, or use the FTP install option.