Hi Jena,
If you are using maven, your IDE should allow you to get all the sources and java doc from your project dependencies.
The source code for jBPM 5.3 in github are here:
https://github.com/droolsjbpm/jbpm/tree/5.3.x
If you want the master branch, latest source code: https://github.com/droolsjbpm/jbpm/
Cheers