With version 3.0, regular WordPress and WordPress MU are merged into WordPress 3. All betas of  WP 3.0 had Site Admin section in backend (even if upgraded from regular WordPress), but since RC and final release this option disappeared. It is a hidden part of WordPress 3.0 and has to be activated manually with a a few edits to a WordPress installations wp-config.php file. You need FTP or shell access to do it.