Instructions

The following instructions are exemplary with the z600 driver. The other four drivers work analogous.
[edit]
Part One - Installing the driver

Get the ebuild, save it to one of the following that matches your printer (see above):

(if you have a Portage Overlay, then you probably want to put the ebuild there)

/usr/portage/net-print/lexmark-z600/lexmark-z600-1.0.1.ebuild
/usr/portage/net-print/lexmark-z35/lexmark-z35-2.0.1.ebuild
/usr/portage/net-print/lexmark-z55/lexmark-z55-1.0.1.ebuild
/usr/portage/net-print/lexmark-z65/lexmark-z65-1.0.1.ebuild
/usr/portage/net-print/lexmark-z700/lexmark-z700-1.1.1.ebuild

digest and emerge it in like manner with one of the following:

emerge --digest -av lexmark-z600
emerge --digest -av lexmark-z35
emerge --digest -av lexmark-z55
emerge --digest -av lexmark-z65
emerge --digest -av lexmark-z700

Restart CUPS:

/etc/init.d/cupsd restart