To see and open the "Manage Web Users" screen, you must be a member of the "Operator" or "Database Setup" role.
.The "Manage Local Users" screen is where these roles can be setup. If you don't see this screen either, you need to ask your administrator to either give you the proper permissions, or to delegate this task to you by making you a member of the "Security Admin" role. This will enable you to "Manage Local Users" yourself.