PyFFI is now using git instead of svn. If you are on windows, you'll need msysgit which you can get from
http://code.google.com/p/msysgit/
To get the latest source code, start a shell (or, the Git Bash on windows), and type
git clone git://github.com/amorilia/pyffi.git
The simplest way to contribute to pyffi, is to fork it (yes! merging is easy so forking is encouraged!) for instance on github:
http://github.com/amorilia/pyffi/
and let me know if you want me to pull your changes (github has all the features to make this as painless as possible).
For more information about git, here's a good guide: http://github.com/guides/home