Sorry about any confusion. The Pixhack kit does not come with a USB cable, or a separate external USB extension. It does however include external LED and USB module which connects to the I2C splitter and the extension ports as shown in the image below.
If you can let me know what R/C receiver you are using I can offer some help on how to get it connected to the pixhack. However if your receiver has PPM or SBUS out, then you can plug it directly into the PPM port of the pixhack. Otherwise if your R/C receiver outputs PWM you will need to use the included PWM to PPM encoder as shown below: