Alvaro is right.
The correct file is//volume1/@appstore/Tomcat7/src/conf/tomcat-users.xml, edit it with putty or winscp.
Content example:
<role rolename="admin"/> <role rolename="admin-gui"/> <role rolename="manager"/> <role rolename="manager-gui"/> <role rolename="manager-script"/> <role rolename="manager-jmx"/> <role rolename="manager-status"/> <user username="user" password="password" roles="manager-gui,manager-script,manager-jmx,manager-status,manager,admin,admin-gui"/>
Stop and run the Tomcat package after making changes to the configuration file to ensure that Tomcat reloads it.
source share