The fastjet-contrib space is intended to provide a common location for access to 3rd party extensions of FastJet.
After downloading and unpacking, enter the fjcontrib-1.038/ directory and then run
./configure [--fastjet-config=FILE] [--prefix=...] [...]
A contribution named "SomeContrib" is usually accessed by
including "fastjet/contrib/SomeContrib.hh" in your C++ file, and
linking with -lSomeContrib.
As of 2018-09-23, developer access is still broken because of the HEPForge upgrade. The FastJet authors are doing what they can, but they have not yet been given write access to the underlying svn. In particular the instructions below do not work.
The old process for those who wished to develop their own contribution or submit new ones was to use svn to checkout the contrib framework,
svn checkout http://fastjet.hepforge.org/svn/contrib/trunk fjcontrib
then run a local script to get the current set of individual contribs