This is usually the case when you are trying to run the application with a Java version that is less recent than the one the designer was built with. The designer currently requires Java6 to run, so make sure your application server is running using Java6. Let us know if this helps.
Finally, I read the file: http://anonsvn.jboss.org/repos/labs/labs/jbossrules/contrib/designer/readme.txt, and this indicates that it requires Java 6. But when I make the deployment of the file: designer-1.0.0.026-SNAPSHOT.war, the editor does not run correctly. I try http://anonsvn.jboss.org/repos/labs/labs/jbossrules/contrib/designer/designer.war file, then the editor is run.