You can browse the SVN repository online via Xfce WebSVN. If you want to check out the source code, you can do this using this command:
svn co http://svn.xfce.org/svn/xfce/installit/trunk installit
Please note that the access is read-only. If you’d like to contribute to the development, sign up to our mailinglist first (see the getting involved paragraph on the frontpage).