The link within the Tools option that it is trying to go to is: wpmu-admin.php (Domain Mapping admin page).
You definitely have something wrong with your install. wpmu-admin.php was the network admin dashboard page in WordPress MU (pre WP 3.0).
Try reinstalling your WP files.
Secondly, by network admin I do not mean the dashboard of your main site. There is a link to Network Admin in the toolbar (under the My Sites menu). Once you are in the Network Admin area there is a Settings menu.