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-1.050/ 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.
The process (updated after hepforge update, still under test) for those who wish to develop their own contribution or submit new ones is to use svn to checkout the contrib framework. You will need a hepforge account if you want to commit anything (contact the fjcontrib maintainers with your hepforge username to get write access).
svn checkout https://svn.hepforge.org/fastjetsvn/contrib/trunk fjcontrib
then run a local script to get the current set of individual contribs