Sorry, we documented it in the install.html
https://github.com/krisv/jbpm/blob/master/jbpm-installer/install.html
"To limit the amount of data that needs to be downloaded, we have disabled the download of the Eclipse
BIRT plugin for reporting by default. If you want to try out reporting as well in the jBPM console,
make sure to put the jBPM.birt.download property in the build.properties file to true before running
the installer."
Unfortunately, it seems it wasn't copied to the installer chapter in the documentation, so I'm adding that now.
Kris