Fatal Error - Pro Sites

Just purchased the pro sites plugin and got a fatal error.

Plugin could not be activated because it triggered a fatal error. "Fatal error: Call to undefined function switch_to_blog() in /home/siteflip/public_html/wp-content/plugins/pro-sites/pro-sites.php on line 196"

I cannot open the pro-sites.php file as it breaks my browser trying to load it (must be a big file)

Any ideas why I am getting this error or how to fix it?