I'm using eclipse 3.3.2 so I had to go for GEF 3.3 and Green 3.0. When dropping the plugin in the plugins folder (GEF I installed using the update sites) I had the options in the eclipse menu added to create UML diagrams.
Only when trying to view a generated Green UML file, extension .grn I got an error from eclipse:
Problems occurred when invoking code from plug-in: "org.eclipse.jface".
You can view the error log by going to the plug-in development perspective where at the bottom there is a tab Error Log (next to Tasks). I found:
Google gave me 2 threads on this problem: http://fusesource.com/forums/thread.jspa?messageID=1275 and http://www.eclipsezone.com/eclipse/forums/m92238620.html.
Solution was to update the com.ibm.icu_3.6.1.v20070906.jar in the plugins directory with a more recent version, like: com.ibm.icu_3.8.1.v20080530.jar. You can find this update at http://download.eclipse.org/tools/orbit/downloads/drops/R20080807152315/.