This BB doesn't appear to have "invisible mode" option enabled by the administrators.
The vBulletin site, where the manual for this BB is posted, shows the invisible mode option under "adding or editing a user" .. but I certainly can't see it enabled under "edit options" on this BB ..
Perhaps the BB is set up, so the ''invisible mode" option, only appears when you're doing a new registration ..
vBulletin manual ..
http://www.vbulletin.com/docs/html/
The vBulletin website community forums offer a nearly limitless amount of assistance ..
http://www.vbulletin.com/