I am trying to activate the Multi-DB on multisite wp

I am trying to activate the Multi-DB on multisite wp installation (on the main blog) however i got this error:

Plugin could not be activated because it triggered a fatal error.

Warning: require_once(/home/xxx/public_html/wp-content/db-config.php): failed to open stream: No such file or directory in /home/xxx/public_html/wp-content/plugins/multi-db/db.php on line 140