Here is the source code. Two include files are needed: HERWIG64.INC and herwig6400.inc.
Here is the documentation and and release note (text or postscript).
Here is a simple test job that generates QCD 2->2 events at the Tevatron.
There is also a test job that shows how to read in SUSY input files and generate SUSY events at the LHC. To run it you will need the file sugra_pt2.in from the ISAWIG web page. Note that it produces a few warning messages when the decay tables are processed. This is due to the fact that both the built-in decay table and the one read in from file contain entries for top decays, and is not a problem: the duplicates are simply deleted.
IMPORTANT NOTE: All of the above files are held on an NT server. Depending on what kind of machine/browser you are using, the DOS control characters might not get converted properly. I'm sure there must be a way around this, but as a temporary measure, all of the above files are also mirrored on a unix machine at CERN.