Run in the plugin folder: git clone git://github.com/joshsten/JChat.git
Run in the main redmine folder: rake db:migrate_plugins RAILS_ENV=production
The plugin should be installed. This plugin hasn't been throughly tested on a clean install. Let me know if you encounter bugs.
A minor annoyance is the loading ajax pops for about half a second every time a refresh is requested on a clean redmine install. I think I can disable this as I believe the timetracker plugin does something similiar.