The main branch of development is "trunk", which is the folder directly under the above URL.
You can also browse the Subversion source here.

Building
Building requires you to have Maven2 installed. Once you have downloaded the source, you simply run "mvn clean install" in the top-level directory (directly under trunk in svn).
When adding Wicket RAD to the build of your own project with Maven, you only need to define a dependency to wicket-rad-core, as this will include all other dependencies.

Eclipse & other IDE's
You may find that you have build problems directly from source in Eclipse or other IDE's. If you do, simply rebuilding the Eclipse project files with maven by running "mvn eclipse:clean eclipse:eclipse" should do the trick. If you use any other IDE, please refer to whatever Maven targets are available for it.