I use svnx to access the SVN repository. Nice GUI and easy to use.
http://code.google.com/p/svnx/
Just put in the InstaDMG SVN address into svnx: http://instadmg.googlecode.com/svn/trunk/
Once you connect to the repository you just select the revision you want and press the ‘Checkout’ button in the top left. Then save it to somewhere on your HD.