how do i download it
If you click on the image in the middle of the homepage, or if you click on either of the two links shown in
that will launch the software using "Java Web Start". This takes care of downloading the Java code for the software (which is a .JAR file), and will also make sure that the appropriate 3D library (another .JAR file - the library is called "JOGL"). This is the best way to start the software running.
If you want to download the JAR files or the source code *directly*, look at the project's "File Releases" page: https://sourceforge.net/project/showfiles.php?group_id=122371 - if you do this you'll also need to make sure you install the JOGL library, available from http://jogl.dev.java.net/