Early versions of the XTL did not function with USB until you upgrade the firmware. That may be the problem. If so, you will need to connect with MIDI cables to update the XTL. After that you can use USB.
indeed: please check the current installed Firmware version: press OUTPUT MODE/SYSTEM, then turn SELECT all the way clockwise
the LCD will show 'POD XT LIVE v. XX'
if the shown version is later than 1.01, you can use your USB connection
see http://line6.com/support/docs/DOC-3827 under "Q: My unit cannot update via USB, and I have Flash Memory 1.0x. How do I update from Flash Memory 1.0x to a newer version?" and http://line6.com/support/docs/DOC-4287
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