Google Code no longer allows for downloads, therefore you will have to use SVN to get v2.3.1:
svn checkout http://box2d.googlecode.com/svn/tags/v2.3.1
You can view the changes here: https://code.google.com/p/box2d/source/list
The main change was the conversion of the Testbed to use GLFW and imgui instead of freeglut and GLUI. GLUI was not supported and I was having trouble with freeglut in newer versions of Visual Studio. If you work in Visual Studio 2013 or Xcode 5, you can open the project file and get started. On other platforms you will have to do more work to set things up. The big difference in running the Testbed is that the working path needs to be setup correctly so that the font may be loaded. In Visual Studio you can set the working path under the project settings. In Xcode you can setup a custom working directory in the Testbed scheme. The Testbed will crash if it fails to load the font file.