http://localhost:8080/manager/html gives a 404 error on apt-get install of tomcat6 (6.0.28 on JVM 1.6.0_20-b20 on 2.6.35-27-generic amd64).

http://localhost:8080/host-manager/html works. Lists one Host name, localhost.

Installed tomcat6-admin with apt-get.

ls dpkg -l | grep -i tomcat6-admin

ii tomcat6-admin 6.0.28-2ubuntu1.1 Servlet and JSP engine -- admin web applications

$ cat /usr/share/tomcat6/conf/tomcat-users.xml

<tomcat-users>
<role rolename="admin"/>
<role rolename="manager" />
<user username="tomcatuser" password="Password1" roles="admin,manager"/>
</tomcat-users>

cat /usr/share/tomcat6/conf/Catalina/localhost/manager.xml

<Context path="/manager" 
    docBase="/usr/share/tomcat6-admin/manager" 
    antiResourceLocking="false" privileged="true" />
<role name="manager" />
<user name="manager" password="Password1" roles="manager" />
<user name="tomcatuser" password="Password1" roles="manager" />

Those two files are the only documentation I've seen on how to setup the Manager webapp, and they seem to be compliant with the requirements.