Downloading the Library
There's only one file that you need to download. It's a JAR file, which means that it should work on any reasonable operating system. However, you'll need Java 8 installed. If you plan to use Java to write your agent, download JDK 8 from Oracle's website. If you're using Python, you'll only need the JRE. You can get that here.
Once you've got the file, your next step will depend on which language you're using.
Running the Library
You can run this file from the command prompt or terminal by executing the following command:
java -jar SnakeRunner.jar [-j path/to/java.jar] [-e path/to/executable] [-p path/to/python.py] [-h <keys>|normal]
So let's say we want to run a game, providing one Python file, one JAR file and one human player who will use the use the WSAD keys. We would accomplish this with the following:
java -jar SnakeRunner.jar -p my_agent.py -j my_agent.jar -h wsad
This will then begin a new game which will be contested by the three provided players, and a single built-in agent.