This is a repo of the USB Serial code for the Teensy microcontroller code from http://www.pjrc.com/teensy/usb_serial.html. It was forked at version 1.7 and contains (or will soon contain) some small improvements to make it easier to use from within an event loop.
Check out this repo into a sub-folder of your Teensy project (e.g. usb_serial/...) Edit the SRC line of your Makefile to include usb_serial.c, e.g.:
SRC = $(TARGET).c \
usb_serial/usb_serial.c
When including usb_serial.h in your source files, qualify it with the path to the sub-folder, e.g.:
#include "usb_serial/usb_serial.h"