Birt 3.7 is a little different in that you no longer need any directories or jars ourside of the war. Please take a look at the jbpm-installer build.xml to see how it sets it up for AS7 and AS7 ( https://github.com/droolsjbpm/jbpm/blob/master/jbpm-installer/build.xml - for AS7 specifically: https://github.com/droolsjbpm/jbpm/blob/master/jbpm-installer/build.xml#L410 ).
If you are using the jbpm installer the birt setup is all done for you.
Hope this helps.