That's not totally true.

The original version of the firmware (v 1.0) you could only calibrate via ethernet, however it's been 'corrected' in later revisions, and you can now calibrate via USB.

Your unit might already have the firmware needed... Unplug the FBV, and hold down the 'D' footswitch while plugging back in. It should come up and say 'Calibrate' on the FBV screen. There's a video from Line 6 on youtube showing the rest. Ignore the fact that he uses ethernet, it will work the same over USB.

If it doesn't say 'calibrate' and instead just shows normal midi information, you must have the old firmware. Install Line 6 Monkey, connect the FBV, and update the firmware to the latest, then try again.

I was pretty upset at first because I couldn't calibrate my newly aquired used unit. But now that it's calibrated (over USB) it's the best thing I've bought since Guitar Rig 5!

If you want some help setting it up for Guitar Rig, hit me up! There's a pretty good video on Youtube for it, but there are a few things that bug me about that guy's setup. His switch lights don't stay lit when the effect is on for instance.