I now understand more. The human task service from the demo is just an example. For integration of your own logger you could follow these steps:
build.xml:
<fileset dir="${install.home}/runtime" includes="*/.jar"/>
<fileset dir="${install.home}/db/driver" includes="*/.jar"/>
<fileset dir="${install.home}/task-service/target" includes="*/.jar"/>
</path>
Then download bindings from:
http://www.slf4j.org/download.html
Copy the slf4j-simple-1.6.1.jar (for example - it is the most simple logging) to the target folder.
Now if you "ant start.human.task" you do not see any error messages but lot of logging.