You cannot do it with conditional formatting unless your "buttons" are pure text--which means they won't look like typical buttons. Conditional formatting can change text size to make text appear/disappear and can control fill color settings, but not the appearance of borders around the button objects.
You can make them appear and disappear, but you'd need to use other means. Extra layouts is simplest to set up (I know, not what you want to do here).
You can put the nav buttons inside an invisible tab control with two panels--one with the buttons and one empty. By giving each panel an object name, go to object can select a panel in the tab control to make the buttons appear and disappear.
You can put the buttons inside a portal and play games with portal filtering or the value of the fields in the relationship to control whether or not a related record appears in the portal. If there's a related record in the portal, you see your nav buttons. If there is no related record, they have disappeared.
Thanks phil, i will give them a try!!