In Arduino, click Help > About to check which version you have.
If older than Teensyduino 1.35, get the latest from the website and install. If you have an old version of Arduino (pay attention to the list of supported versions when you start the installer), you'll need to get the Arduino installer from
www.arduino.cc also. Update Arduino first, then install the new Teensyduino.
When you run the Teensyduino installer, leave all the libs selected. If you don't install SerialFlash, then SerialFlash.h will be missing!