Are you just using the admin console that came w/ AS 5.1 or did you try to deploy your own version of admin-console.war?
Thanks to take intrest.I am using the default one only.
See if the plugin jar file rhq-platform-plugin-1.3.0.EmbJopr.1_2_0-1.jar exists under deploy/admin-console.war/plugins/. If so, run jar tvf on it to verify that it is not corrupt. If it's missing or corrupt, I would redownload AS 5.1 from sourceforge and reinstall it from scratch. If the file exists and is not corrupt, something else you could try is to stop AS, blow away its working directories (cd server/default; rm -rf work tmp data logs), and then restart it.