WHMCS WP INTEGRATION fatal error

This is the error I receive when I try to activate the plugin:
Notice: Use of undefined constant WHMCS_TEXT_DOMAIN - assumed 'WHMCS_TEXT_DOMAIN' in /home/canwebservices/public_html/wp-content/plugins/whmcs-wordpress-integration/whmcs-integration.php on line 38
The WHMCS WordPress Integration plugin requires the PHP mbstring extensions.