for 5.4 it's enough to run ant start.demo or ant start.demo.noeclipse to initiate demo installation of jbpm. It start JBoss AS and all application deployed to it. Next examine server.log to check if there were any problems starting any of the applications.
the one that you executed start Mina based task server and the error you see is due to missing path to role.properties file that defines group membership for users in task service. You can add additional jvm parameter into that target in build.xml that will inform about location of that file:
jbpm.user.group.mapping=ABSOLUTE_PATH_TO_ROLES_FILE
HTH