Arquillian did not only change place where source code is storaged - it did change hole type of version control system. Arquillian uses git now.
To get started quickly, you could checkout by using TortoiseGit: http://code.google.com/p/tortoisegit/. Works almost the same as TortoiseSVN does.
Hint for integrated usage within IDE:
- Eclipse: use http://www.eclipse.org/egit/
- IDEA: has native git support
And of course you can use git by command line.
BTW, if you not prefer learning a new version control system, you can simply browse to github repo and download the sources directly.