If you are interested in seeing the trace messages of the PDP engine in JBossXACML, you will need to add the following line to the logging.properties file in use for your setup.
# Set the default logging level for the logger named org.jboss
org.jboss.security.xacml.sunxacml.level = FINEST
If you are running it in Apache Tomcat, then the file is in conf directory.
In JBoss Application Server, you should just add a trace category in the conf/jboss-log4j.xml
The trace messages will be found in the logs of your server.