I have run into this problem a couple of times as well including yesterday when creating clean setup.
In order to use the interface to manage the wiki you need to create an admin (I'll use root).
So you must edit conf/local.php. Go to the line
$conf['superuser'] = '!!not set!!';
And set it to a user name such as
$conf['superuser'] = 'root';
Then if root already exists login as root and the admin button or link will be there.
If 'root' does not exist register as root and then the admin button will be enabled.
Later you can create an admin group for root and other administrators then change the line to
$conf['superuser'] = '@admin';
Hope this helps!
Mike