Sencillo, solo agrega esto en /etc/tomcat6/tomcat-users.xml
>>>-------------------------------------------------------------------------------------------------<<<
<role rolename="manager-gui"/>
<user username="tmanager" password="s3cr3t" roles="manager-gui"/>
<role rolename="admin-gui"/>
<user username="tadmin" password="s3cr3t" roles="admin-gui"/>
>>>-------------------------------------------------------------------------------------------------<<<
Yo estoy usando tomcat7, solo agrega esto para crear un usuario para cada role puedes cambiar el usuario y el password a tu gusto, luego solo reinicias y listo:
root@rojoblandino:~# cat /etc/tomcat7/tomcat-users.xml
<tomcat-users>
<!--
NOTE: By default, no user is included in the "manager-gui" role required
to operate the "/manager/html" web application. If you wish to use this app,
you must define such a user - the username and password are arbitrary.
-->
<!--
NOTE: The sample user and role entries below are wrapped in a comment
and thus are ignored when reading this file. Do not forget to remove
<!.. ..> that surrounds them.
-->
<!--
<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"/>
-->
<role rolename="manager-gui"/>
<user username="tmanager" password="s3cr3t" roles="manager-gui"/>
<role rolename="admin-gui"/>
<user username="tadmin" password="s3cr3t" roles="admin-gui"/>
</tomcat-users>