None of the guitar's sound is transmitted through the Fretlight's USB cable to the computer. All the sound from the pickups go through the guitar's 1/4" output jack, through a music cable, just like any other electric guitar to an amp. To hear that output through your computer there are a few options. Some of which are listed below.
1. If your sound card has an 1/8" input/mic jack, buy a 1/4" to 1/8" adapter and try to capture the guitar as you would a microphone. This will most likely not be the best quality, and you may not hear anything because the signal does require some amplification to be heard correctly. Some sound cards actually have a pre-amp built in. If it does not plug your guitar into your amp and use the amp's headphone or aux out to connect to your computer's input.
2. Use a USB interface device that accepts an instrument's audio cable connection.
3. Use a mixer that has a built in preamp to boost the signal of your guitar and then plug it into your computer's mic input.