1. Unpack the FlexamySynh.rar archive to your Arduino/libraries folder. Make sure the target folder is Arduino/libraries/FlexamySynth.
2. Enable versioning support in the library:
2a. If You use Arduino older than 1.0, move the provided 'Arduino.h' file to your Arduino/hardware/arduino/cores/arduino directory.
OR
2b. If You use Arduino 1.0 or newer, add the following entry to your Arduino/hardware/arduino/cores/arduino/Arduino.h file:
#define _ARDUINO_VER 0x0100
and delete the provided stub version Arduino.h.
3. Optionally, if you use Arduino 1.0 or newer and already have the NewSoftSerial library installed, you may delete the provided version, and use your pre-existing one. (untested)