Due to licensing issues, we can't redistribute the Microchip USB source. 
You can get it from the Microchip website for free: 
http://www.microchip.com/usb

1.Download and install the USB source. These files install to c:\Microchip Soultions\ by default.
2.Place the project folder in the Microchip source install directory (c:\Microchip Soultions\ by default)
3.Copy usb_descriptors.c from \Microchip Solutions\USB Device - CDC - Basic Demo\ to this project folder.
4. That's it. You've got the latest source and we're compliant with the license.

Depending on the install location you may need to tweak the include paths under Project->build options.