Firstly do you understand that you need to open the xbox and solder the lpt cable to the mainboard...
Secondly, the lpt cable is not for jtagging. It's for reading and writing to the nand which basically contains your main dashboard. (Bootloaders, NXE, config files, ...)
You will have to replace this dashboard with the hacked dashboard called xbr. Jtagging is necessary for this all to boot.
The jtag points are located on following pictures:http://imgur.com/Fdjmi.png
: For Xenon CPU'shttp://pictures.xbox-scene.com/xbox360/free60/diagram.jpg
: For Falcon, Zephyr, Opus & Jasper
Off course this all is only possible if you haven't updated with the august 2009 update which blows the efuse of the vulnerability hack.
If this all doesn't make sense to you, I'll suggest you start reading or just find someone who can do the hack for you!!!!
If you're searching a place to start try: www.free60.org