SYSTEM REQUIREMENTS

In order to run Robocode, Java 5.0 Standard Edition (SE) or a newer version of Java must be installed on your system. Both the Java Runtime Environment (JRE) and the Java Developer Kit (JDK) can be used. Note that the JRE does not include the standard Java compiler (javac), but the JDK does. However, Robocode comes with a built-in compiler (ECJ). Hence, it is sufficient running Robocode on the JRE.

Also note that it is important that these environment variables have been set up prior to running Robocode:

JAVA_HOME must be setup to point at the home directory for Java (JDK or JRE).
Windows example: JAVA_HOME=C:\Program Files\Java\jdk1.5.0_22
UNIX, Linux, Mac OS example: JAVA_HOME=/usr/local/jdk1.5.0_22

PATH must include the path to the bin of the Java home directory (JAVA_HOME) that includes java.exe for starting the Java virtual Machine (JVM).
Windows example: PATH=%PATH%;%JAVA_HOME%
UNIX, Linux, Mac OS example: PATH=${PATH}:${JAVA_HOME}/bin

You can read more details from here:

System Requirements

If you want to program robots in .NET or control Robocode from a .NET application, you need to install the Robocode .NET API plug-in on top of Robocode. The plug-in is installed by double-clicking the robocode.dotnet-xxx-setup.jar the same way as Robocode itself is installed.