cant pro-sites.php on line 191

can't Activate pro site plugin

it say...

Fatal error: Call to undefined function switch_to_blog() in /wp-content/plugins/pro-sites/pro-sites.php on line 191

and my wp is a new install one, other plugin can Activate..

it is a problem from i have not define('WP_ALLOW_MULTISITE', true); in wp-config.php?

THANKS!