Elgg code is no longer being hosted in a Subversion repository. However, since we're hosted on Github, you can still get SVN access if you like.
To checkout the Elgg source:
svn co http://svn.github.com/elgg/elgg.git /your/save/path
There are also plugins and themes that can be checked out. For example:
svn co http://svn.github.com/elgg/community_plugins.git /your/save/path