In order to get JMX information out of a running instance of Tomcat under Eclipse, you must add the Sun’s JMX Remote Management debug option to the start-up command of Tomcat.

1. Goto the Servers pane at the bottom of the screen. (Or Window->Show View->Server to open the Servers View.)
2. Open Tomcat Overview configuration window by double clicking on Tomcat v6.0 Server at localhost.
3. Under General Information click the Open launch configuration
4. Select the Arguments tabs of the Edit launch configuration properties
5. In VM arguments add

   -Dcom.sun.management.jmxremote

6. Click Ok and close the Tomcat Overview configuration window.

I say this is under Eclipse but the same action should be taken when running Tomcat outside of Eclipse.