Install Ant and Java as described in Get Started.

$ svn co https://svn.openqa.org/svn/selenium-grid/trunk Selenium-Grid

Or, alternatively, if you are not leaving in the past and are already familiar with git:

$ git clone git://github.com/ph7/selenium-grid.git Selenium-Grid

$ cd <project root>
$ ant sanity-check

Run the build to make sure that it passes on your box:

$ ant dist

You will then find a new distribution under the target directory (zip and tar formats).