A couple of things to try. In the Print&Scan preference panel click in the window that lists the printers. Right-click (control-click) and select "Reset printing system." Then try to add the printers again.
You should not have downloaded and installed the printer drivers manually. That can screw things up. The location for drivers may have changed in Mountain Lion for all we know. All printer drivers should be installed only through the Print&Scan preference panel. All drivers are now supplied to Apple by the printer manufacturers and are customized for the OS they are being installed on. Just let the Print&Scan panel do its thing.