from packages: --
'pkg_add -r zope-zwiki'
from ports: --
'cd /usr/ports/www/zope-zwiki; make install'

should do it. I'm not sure how fresh these are.

from tarball: --
similar to InstallingOnDebian.

At least prior to freebsd 5, there is a serious python crashing issue which is fairly frequently encountered when using Zope, especially with Plone or Zwiki. To solve it you need to rebuild python with a larger stack size. See #226 on freebsd, python/zope may crash repeatably when browsing diffs or saving certain pages.