Here you can find the javadoc for latest SNAPSHOT: http://hudson.jboss.org/jenkins/view/Drools%20jBPM/job/jbpm/lastSuccessfulBuild/artifact/jbpm-distribution/target/ I'm pretty sure that the APIs didn't change that much!
Thanks Esteban, but your link concerns with the users guide only (html/pdf).
I could not find any javadoc for engine API there.
I would download what I can see online here http://docs.jboss.org/jbpm/v5.1/javadocs/
If you are using maven, maven will download and install the sources and the javadocs for you. Take a look at your IDE in the dependencies configurations you will see that it's just a couple of clicks.