I dont have an idea whats the difference in 3.1 and 3.2. But the following worked for me to use the process designer plugin in eclipse.
Eclipse Version used - eclipse-3.2
jBPM Process Designer plugin - jbpm-jpdl-designer-3.1.0alpha2
I have just copied the features and plugins from the jbpm-jpdl-designer-3.1.0alpha2.zip into corresponding folders in eclipse.
Then started eclipse.exe from command line using the -clean option.
Now I was able to see the options for new process designer project etc.
See If this can be of some help to you.