use AdSharing plugin on WP3.0 (single site mode)

i just tried to run the Ad Sharing plung on an WP3.0 site (in single site mode) but when i activated the plugin i get an

Fatal error: Call to undefined function is_site_admin() in /is/htdocs/wp27273_5KN6DZZ224/www/wp/wp-content/plugins/ad-sharing.php on line 59

error.
i´d like to use it on that single blog installation with multiple autors, as i read in the plugin description this should work (as it is compatible to WP3.0), right?

any ideas?
tnx, eric