To install a released version, download the required tarball from the KtJet download page. Unpack it and cd to the directory that has been created. Then follow the usual procedure:
./configure make make install
From the repository
If you have a developer account you can check the code out from the Subversion repository using e.g.
svn co svn+ssh://svn.hepforge.org/hepforge/svn/ktjet/trunk ktjet
If you don't have a developer account you can still get read-only access via HTTP:
svn co http://svn.hepforge.org/ktjet/trunk ktjet
Run autoreconf -iv (using autoconf version 2.59 or higher) and then do configure etc. as described above.