fatal error

Fatal error: Call to undefined function is_multisite() in /home/content/47/8061447/html/clients/blr/wp-content/plugins/wp-maintenance-mode/wp-maintenance-mode.php on line 215

THis is the message that comes up after i installed the pluging. i cannot get to dashboad and this is what comes up for any type of entry

http://www.breathliferadio.com

Thank you for your help

http://wordpress.org/extend/plugins/wp-maintenance-mode/