The following prerequisites must be fulfilled for Applications Manager to monitor Tomcat servers: 

1. 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://host:port/manager/html. Try <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.
2. The user role required to access the server must be manager for versions 5.x & 6.x and manager-jmx for version 7.x and above. To add a role as "manager" / "manager-jmx" for any of the users such as tomcat, role1, or both, you need make changes in tomcat-users.xml file located in the <TOMCAT-HOME>/conf directory. 

For Example:

For versions 5.x & 6.x :

Default configurations in tomcat-users.xml in Tomcat Server.

<tomcat-users>

<user name="tomcat" password="tomcat" roles="tomcat" />

<user name="role1" password="tomcat" roles="role1" />

<user name="both" password="tomcat" roles="tomcat,role1" />

</tomcat-users> 

After adding the roles for the "tomcat" user, the modified entries will be as follows:

<tomcat-users> 

<user name="tomcat" password="tomcat" roles="tomcat,manager" /> 

<user name="role1" password="tomcat" roles="role1" /> 

<user name="both" password="tomcat" roles="tomcat,role1" /> 

</tomcat-users> 

For versions 7.x and above: 

The modified entries will be as follows:

<tomcat-users><user name="tomcat" password="tomcat" roles="admin-gui,manager-gui,manager-jmx"/><user name="tomcat1" password="tomcat1" roles="manager-gui,manager-jmx"/></tomcat-users>

On making the configuration, restart the Tomcat Server.

Now, when adding a new Tomcat (5.x, 6.x and 7.x) monitor, specify the username/password of the user configured with role "manager" (5.x & 6.x ) / "manager-jmx" ( 7.x & above ), when discovering the Tomcat Server.