Grab the plugin at http://rubyforge.org/frs/?group_id=2978
and drop it in your vendor/plugins directory.
This cells-1.0 release runs with both Rails 1.2.x and Rails 2. Just be sure to checkout the latest Engines version from the engines repository, since cells depends on this fine plugin.
Leech the latest cells code from svn:
svn checkout svn://rubyforge.org/var/svn/cells/cells_plugin/trunk cells