google code uses svn repositories so install any svn client, and checkout the code if you are using linux or mac, svn may already installed, then you can use command "svn checkout <path to source trunk>" in terminal
use "svn --version" to confirm svn installation (mac or lnux)
you can find list of svn clents here
http://subversion.apache.org/packages.html#windows
you can find repository url in "source" tab in google code project.
EDIT:
some projects allows us to download source as single archive if so you can download them in download tab. but most of projects does not offer that feature.