Look at this thread:http://en.forum.laptop.org/viewtopic.php?f=1324&t=180103#p615973
It is not just a serial cable. You must have a serial adapter that converts (supplies) the signals at 3 volts or a USB to serial adapter (3.3 volts).
If you can not fabricate the final connector to the XO port, send me a personal message for an improvised connector. It is really a simple process using a PC as terminal, if you have the adapter/cables/PC drivers and you are capable of some disassembly to remove the back of the XO (which requires removing some screws from the front).
Don't forget to charge the internal RTC clock battery by leaving the XO on for several hours (eventhough it does not boot), if it is discharged.