See the online help section on external tasks - particularly the section regarding security considerations.
I suggest you put your "java -jar jmxtools.jar ........." command in a batch/file and drop this in the config/commandarea folder. Use the name of the batch file/script as the 'command' parameter of the executable external task.
"C:\Program Files\Java\jre6\bin\java" -jar jmxtools.jar runjob from your local machine first
Make sure you execute this command in a directory where jmxtools.jar is located,also password for dnadmin is missing in the command