Take a look at this video, it should help: http://vimeo.com/26126678.
If you are using jBPM 5.1 you should definitely upgrade to Designer 2.0 ( http://sourceforge.net/projects/jbpm/files/designer/designer-2.0/ ) or get the latest from master from people.redhat.com/tsurdilo/designer/master. Docs are here: http://docs.jboss.org/jbpm/v5.2/userguide/ch10.html
If you have any specific questions, just ask.