This key can be obtained at members area. The key is unique for each license. You can use one key only once.
Free register key can be obtained at this page. The key will be valid for 30 days from the moment of the receival. Only one trial key can be used at a domain.
Example:
In case you enter CNStats administrative interface at www.cnstats.server.com/cnstats/ or at cnstats.server.com/cnstats/, then the register key should be obtained for the domain cnstats.server.com and $STATS_CONF["cnsoftwaredomain"] parameter in config.php file should be set as follows:
$STATS_CONF["cnsoftwaredomain"]="cnstats.server.com";