This library is primarily design to be used with the Arduino IDE and therefore has a simplified setup procedure for that purpose. If you don't use the Arduino IDE, please read Using the library without Arduino.
Download the zip package and extract it to:
<your Arduino Sketch folder>/libraries/ArduinoJson
Then restart the Arduino IDE.
Click File / Example / ArduinoJson.
Just add the following line at the top of your program:
#include <ArduinoJson.h>
Then follow the instructions: