According to the wiki: http://wiki.apache.org/spamassassin/MassCheck
mass-check lives in the masses of the source tarball. This was true for SA 3.1.x and older, but 3.2.x no longer includes it.
It can still be grabbed SVN, or even the web interface to SVN, but the wiki should point to the correct source for it.
The big question is, where do the devs think folks should go to get it?
