At first, ZK looks for this file in the classpath. If not found, it looks for the conf directory.
Application Server
Location
Tomcat
Place i3-log.conf under the $TOMCAT_HOME/conf directory
Others
Try the conf directory first. If not working, you could set the system property called the org.zkoss.io.conf.dir directory to be the directory where i3-log.conf resides.