You should also specify which profile you want to use in the url. (adding ?profile=drools at the end of the url should work)
Or if you use it in combination with Guvnor, that will do that by default for you.
There is some new documentation on the designer as well:
https://hudson.jboss.org/hudson/job/jBPM5/lastSuccessfulBuild/artifact/target/jbpm-5.1-SNAPSHOT-docs-build/jbpm-docs/html/ch12.html
Kris