I have the same opensuse9.3 pro and dell 720/lexmarkz615 . i surfed sites and found this guide which helped me:
http://paulvoogen.tripod.com/id1.html
i am not well versed in command line, i like running guis more but it worked for me.
i put the original downloaded tar z600 in /home/user/lexmark folder ('user' being a your local user folder, creating an additional 'lexmark' sub-folder)
i unzipped it then ran the shell execution command:
sh z600cups-1.0-1.gz.sh -keep -target tmp
i did get an libtcl8.3.so error but i ignored it
i went into ./tmp and ran back to back shell execution commands:
rpm -ivh z600llpddk-2.0-1.i386.rpm
rpm -ivh z600cups-1.0-1.i386.rpm --nodeps
when i installed suse9.3 it already saw the printer but i didnt have the proper driver (which btw answers your question why is the reason it sees it but cant print to it, the lack of a proper driver)
i went to Yast2 > Hardware > Printer
then at the bottom I 'CHANGE' the information to the already installed dell 720
i 'EDIT' the information; select the 'PRINTER MODEL' and 'EDIT'; then look under Lexmark for the Z600 Color Jet Printer
worked for me
btw, pauls site is in another language so i had to pick it apart to understand what to do
p.s. make sure the printer is manually turned on before you test print