This Cosa library is an implementation of the MQTT V3.1 Client Protocol.
To use this library you must download and install the [Arduino IDE] (http://www.arduino.cc/en/Main/Software) (or GCC AVR toolchain) and [Cosa] (https://github.com/mikaelpatel/Cosa).
Download and unzip the Cosa-MQTT library into your sketchbook libraries directory. Rename from Cosa-MQTT-master to MQTT.
The MQTT library and examples should be found in the Arduino IDE File>Sketchbook menu. Open the CosaMQTTclient example sketch. Select the Cosa core by selecting one of the Cosa boards in the Tools>Board menu.