The port you're going to use is the round Appletalk/RS-422 serial port. Not the ADB port whioch is the same size but with a different pin configuration. Turn off Appletalk in the appropriate Control Panel.
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I can't remember, but there usually is a button configuration on a printer that you hold down while turning on the power switch, which forces the printer to print a test page. Your heads are probably dried up in the printer, however, perhaps to the extent that it cannot be cleaned with the cleaning process (which simply tries to force ink through the jets). You can get a printer head cleaning solvent and try to loosen up the dried heads. Don't try to clean them physically, as it is very easy to destroy them.