Thanks for the feedback.
Note that this is only necessary if you have MyEclipse installed because some plug-in in MyEclipse hijacks the *.xml file extension.
Thanks for helping me fix the editor visibility problem. I was struggling with that issue for a couple of hours. I followed your instructions but I did it a little differently. I associated gpd.xml with the jbpm Graphical Process Designer. I'm guessing that approach will not interfere with the xml file associations in other plug-ins and perspectives. Thanks again.