You should install GPD once more,because your plugin is not properly installed.
Here is a more helpful answer...
First of all, I'm not sure that GPD is compatible with Eclipse 3.2. But you can try this and see if it works. Go into Eclipse's File Associations and associate the gpd.xml and processdefinition.xml files with the GPD plugin. You might restart Eclipse after that just for good measure. Let us know if that works.
I am using Eclipse 3.2 with jBPM GPD 18.104.22.168 and it's working fine. You just have to associate the process definition with the GPD, as mentioned above. To do so select Window -> Preferences -> General -> Editors -> File Association in Eclipse and add jBPM GPM as the default editor for the file type processdefinition.xml.
Setting the file association did the trick!
Thanks to everybody!
go to workspace\.metadata\.plugins\org.jbpm.ui
edit file jbpm-installations.xml:
<?xml version="1.0" encoding="UTF-8"?>
<installation location="/g:/eclipse/configuration/org.eclipse.osgi/bundles/168/1/.cp/" name="jBPM 3.1.1" version="jbpm-3.1.1"/>
with /g:/eclipse/ is real eclipse's path of you. I hope you are sucessesfull