In order to work on the graphical designer in eclipse, you need to do the following:
download a fresh eclipse-3.2 from http://www.eclipse.org/downloads/.
download the jBPM binary
fire up Eclipse
go to the update manager and install the Web Standard Tools (org.eclipse.wst)
set up the jBPM cvs repository, details of which can be found on the downloads page
check out the project jbpm.3/designer/jpdl/org.jbpm.gd.jpdl
In order to test, just choose 'Run | Run as | Eclipse Application'
When setting up a new project, you will need to select the directory to which you unzipped the jBPM runtime binary.