If running this command :
./script/plugin install engines
you get the message:
Plugin not found: engines
Is because none of the server listed in your plugin sources has the engine plugin.
So, open the file .rails-plugin-sources
vi ~/.rails-plugin-sources
and add this line
http://opensvn.csie.org/rails_engines/plugins/
This source has the engine plugin and many other listed at http://rails-engines.org/