Antigen can be downloaded from the SourceForge site or by following the links to the left to the individual subproject documentation and then following the downloads link. Several downloads are available:
Antigen is a tool that makes it easy to wrap up your Ant build script as an executable jar file with a GUI.
For simple, one-off jobs, this is the file you are most likely to use - see the QuickStart documentation.
Antibody is the library that does the actual work. Antigen bundles this library, Ant, and your Ant build file into a single executable jar. Alternatively you could use Maven or (!) Ant to do the same job as part of your build process. See below on how you might do this.
You can make Antigen part of your build process if you are using Maven, recreating the installer every time you rebuild your project.
An example of how to do this is Antigen itself, which consists of a master Maven project that builds the core Antibody library, then uses this to create the Antigen tool (above), the Source Code Installer (this) and the hello-world installer (below).
Running the Source Code Installer will install the Antigen source code tree on your local machine, so that you can adapt it for use in your own project.
HelloWorld is a trivial application that demonstrates how to use Maven, Antigen and Ant to write a simple installer. You can inspect the source code for this application here (or by running the Source Code Installer), and in particular the underlying Ant Script.
However, for something this trivial you don't really need Ant. If your installation process just consists of copying an executable then you might be better off with an installer such as Izpack.