Your web.xml looks almost exactly like mine. I'd be surprised if that were the problem. Is your struts-core-####.jar file in the WEB-INF/lib directory of your webapp?
How are you getting to the webpage? I know if it defaults to your index.html, you have to redirect it to an action... like so:
<meta http-equiv="Refresh" content="0; url=MainMenu.action">
Are there any other exceptions/messages your getting in your glassfish/domains/domain1/logs/server.log file that might be able to point us in the right direction?
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