When installing an app, theme or when downloading phpFox you are asked to enter your site's License ID and Key. This information is emailed to you when you first purchase a license. If you have lost this information you can retrieve from here.
Access license key from web server
If you are running Neutron (v4 or higher) you can access your site's license information by opening the file /PF.Base/file/settings/license.sett.php.
In that file you will find something like:
The 32 character jumble of letters and numbers is your license ID and Key.