Getting fatal error when I try to install Pro Sites in a new install of WP


I just now installed Wordpress, and added the single line to wp-config.php to enable Multi Sites.

I tried installing the Pro Sites plugin, but when I try to activate it I get a "Fatal Error" -- this is the error message--

Fatal error: Call to undefined function switch_to_blog() in /home/wpsc/public_html/wp-content/plugins/pro-sites/pro-sites.php on line 193

Kindly let me know how to resolve this error