Message Viewer Not Working

I am using SoapUI 5.3.0.  I have a REST Mock Service that shows date/timestamp when a message hits my endpoint.  From everything I've seen a Message Viewer is supposed to pop up showing the contents of the message if I double click the date/timestamp entry; however, it's not working for me.  Is there some option I must first set or something else I need to do to get it to work?