Tomcat 5.5 does not enable the admin or manager access by default. To allow for admin and manager access, the tomcat-users.xml file found in the /tomcat/conf directory. The original file may look like this.

<?xml version='1.0' encoding='utf-8'?>
<tomcat-users>
<role rolename="tomcat"/>
<role rolename="role1"/>
<user username="tomcat" password="tomcat" roles="tomcat"/>
<user username="both" password="tomcat" roles="tomcat,role1"/>
<user username="role1" password="tomcat" roles="role1"/>
</tomcat-users>

This is the modified file, adding manager and admin to the roles and usernames.

<?xml version='1.0' encoding='utf-8'?>
<tomcat-users>
<role rolename="manager"/>
<role rolename="tomcat"/>
<role rolename="admin"/>
<role rolename="role1"/>
<user username="both" password="tomcat" roles="tomcat,role1"/>
<user username="tomcat" password="tomcat" roles="tomcat"/>
<user username="admin" password="admin" roles="admin,manager"/>
<user username="role1" password="tomcat" roles="role1"/>
</tomcat-users>

Restart the Tomcat services, and you now have access to the administrative pages of Tomcat. Recently tested on a CentOS 5.4 install of Tomcat 5.5.

(source)