yacs installation...

I followed the instructions carefully, and have installed many scripts in the past... Still seem to be having problems. At present when attempting to access http://www.citiesamerica.com/yacs I get the error... No article has been created yet! Use the Content Assistant to populate this server.

Fatal error: Undefined class name 'users' in /home/httpd/vhosts/citiesamerica.com/httpdocs/yacs/shared/global.php on line 1212

Any assistance you can provide will be greatly appreciated.

Steve