Open this file in your favorite XML editor:

jakarta-tomcat-5.0.19/conf/tomcat-users.xml

Add these two elements after the last 'role' element:

<role rolename="manager"/>
<role rolename="admin"/>

Add this element after the last 'user' element:

<user username="admin" password="admin" roles="admin,manager"/>

If you have tomcat running, you'll have to restart it.