Sorry, I didn't see that you were also trying to invoke designer through Guvnor (the correct way).
In your case, seems that you are missing some parameters in the URL you are using:
http://localhost:8080/drools-guvnor/org.drools.guvnor.Guvnor/standaloneEditorServlet?assetsUUIDs=HERE_GOES_THE_UUID&editor=oryx
Please try using this URL. If this still doesn't work, please post the error log you are getting in the app server you are using.
Best Regards,