Try to check how Administrator may approve new users first
If you would like to achieve that admin only registers new accounts, under setup_options.php set
Then login as admin and use the regular registration link, likeyour_forums_url/index.php?action=registernew
You can put this link under your admin panel template, or bookmark, whatever... it will lead to registering an account by admin, so specify any kind of username/password/email you would like to put there. Email notification should be sent on this email, if enabled.