Download the .jar file using the button above and save it somewhere. (You may need to right-click the button and choose "Save Link As..." or similar.)
On some versions of Linux, you can just double-click on the file you downloaded and it will run.
If that doesn't work, open a Terminal and type:
java -jar rabbit-escape-*.jar
(If you saved it to a different place, change "Downloads" to the right place.)
(Make sure you type it exactly right, including capitals.)
If the Rabbit Escape window doesn't appear, you may need to install Java: read the next section.
It's quite likely you already have java installed. If not, to be able to play Rabbit Escape you need to install Java version 6 or above.
To find out, open a Terminal and type:
If you see something like java version "1.8.0_91" (plus loads more stuff), then you are all set.
If you see something like No command 'java' found (or similar, possibly with loads more stuff too), java is not installed and you should keep reading.
NOTE: if you are using the OpenJDK version of Java, sound will be disabled. See issue #9 for more info.
If you use Ubuntu, Debian or Mint, install the openjdk-8-jre package by opening a Terminal and typing:
sudo apt-get install openjdk-8-jre
Running Rabbit Escape is almost exactly the same as running Minecraft. This tutorial might help: How to Play Minecraft in Ubuntu
If you use Fedora (and possibly RedHat or CentOS) install the java-1.8.0-openjdk package by opening a Terminal and typing:
su -c "yum install java-1.8.0-openjdk"
If you use Arch Linux, install the jre8-openjdk package.
More info: Java on Arch.
If you're on a different Linux distribution, check the documentation, or download Java directly from Oracle by following the instructions on java.com.