An application named Manager must be running in Tomcat Server Versions 5.x & above for Applications Manager to monitor the server. By default, this application will be running in the server.
To check if it is already running, open the following url in your browser: http://<Tomcat Host>:<Port>/manager/html
Try <Tomcat Host> as ipaddress, dns name and fully qualified dns name.
Also, make sure the following folder is found in tomcat installation : <TOMCAT_HOME>/server/webapps/manager.