The plugin requires the GeoIP binary database (filename GeoLite2-City.mmdb) saved in your plugin directory (postaffiliatepro/plugins/GeoIp/).
You can downlaod the latest version of the GeoIp database from here: GeoLite2-City.
(Don't forget to extract the downloaded file before copying to the plugin directory)
The free GeoIp database has lower accuracy as the payed version offered by MaxMind, but for most of installations it will be enough, because in most cases you are interested in country level information and City level is not so important.
If you want to have more accurate results of IP identification, you will need to buy the GeoCity library from MaxMind and replace the file name of GeoLite2-City.mmdb in your plugin directory for the one you received from MaxMind.
Accuracy of Free GeoIp database: Over 99.3% on a country level and 76% on a city level for the US within a 25 mile radius.
Accuracy of Payed GeoIp database: Over 99.8% on a country level and 81% on a city level for the US within a 25 mile radius.
IMPORTANT NOTE: only data that has been collected after the activation of this plugin will be shown in GeoIP overlay. Older data do not count in the report!