Plugin throws error

I switched to a wp network to use the plugin. Yep, heard its old but lets make it work properly anyway as it is listed here.

When I want to send a message to another user and type in the screen name instead of the username (which I might not know as there is now way to find it out) I get the following error message at the end of the new message page:

Warning: require_once(/home/suppeoc/public_html/_suppe/wp-admin/includes/internal-linking.php) [function.require-once]: failed to open stream: No such file or directory in /home/suppeoc/public_html/_suppe/wp-content/plugins/messaging/messaging.php on line 1094

Fatal error: require_once() [function.require]: Failed opening required '/.../wp-admin/includes/internal-linking.php' (include_path='.:disappointed:usr/lib/php:disappointed:usr/local/lib/php') in /.../wp-content/plugins/messaging/messaging.php on line 1094