To get the latest code onto your computer, open a command-prompt with svn.exe in your PATH environment variable, and run one of the following commands:
svn checkout svn://rubyforge.org/var/svn/ironruby svn checkout HTTP://ironruby.rubyforge.org/svn/trunk/
On Windows, TortoiseSVN is a great plugin for Windows Explorer that you can use as a graphical source control client when working with IronRuby.