The traditional way to check out the source is to check out the whole argouml project from http://argouml.tigris.org/svn/argouml/trunk. This includes the source code, tools, documentation and web-site, i.e. :
svn checkout http://argouml.tigris.org/svn/argouml/trunk argouml
If you only want to build argouml, and don't want anything else, you could just download the source code and the tools:
svn checkout http://argouml.tigris.org/svn/argouml/trunk/src http://argouml.tigris.org/svn/argouml/trunk/tools argouml
If you just want to build the documentation you can just check out the documentation xml files and the tools:
svn checkout http://argouml-documentation.tigris.org/svn/argouml-documentation/trunk argouml-documentation
If you only want to work with the web site, just check out the www directory:
svn checkout http://argouml.tigris.org/svn/argouml/trunk/www argouml/www
Use the Password for the Source Repository.