The Atomic34 project have 2 main artefacts. The first one is an Eclipse plugin used to write you site definition. You could write your definition in a basic text editor, but the Eclipse plugin gives you syntax coloring and auto-completion, which make it easier to avoid errors.
So, if you don't already have eclipse installed, you would have to download the latest eclipse distribution, Eclipse Indigo, at http://eclipse.org/downloads/. Download either Eclipse For Java Developer or Eclipse for Java EE Developer to your liking.
Once you have eclipse running, you have to install the plugin. There is many tutorial on the web on how to install plugins in eclipse, so I won't go into details here, beside telling you that the update site for the plugin is http://iweb.dl.sourceforge.net/project/atomic34/site.xml
Once you have the plugin installed, continue to [GettingStarted].