Use your SVN client of choice to download either the whole development tree, or individual parts of it.
svn co http://svn.agrip.org.uk/trunk/
The above command will get you all of the components being developed (so that's at least the entire ZQuake codebase, AudioQuake, MAUTH protocol tools and the Stats and Servers web application).
If you just want ZQuake and AudioQuake (the minimum requirements to build a release of AudioQuake), you could issue the following commands.
svn co http://svn.agrip.org.uk/trunk/zq-repo/
svn co http://svn.agrip.org.uk/trunk/audioquake/
If you do that, because of the way the build system works, it would be helpful for you to make a trunk/ directory (it can have any name, though) and check out the individual components into it. This will help you later on, when you will need to put things such as engine binaries in places the build system can find.
If you are behind a nasty proxy, this may not work, in which case you can use the URL svn://agrip.org.uk/agrip/trunk/audioquake/ but don't supply any authentication information.
You can update your working copy at any time by issuing the svn up command in any checked-out directory.