well... you didn't go the easy way.
You will find under the download section on www.ispconfig.org
the link to my roundcube plugins.
Which are here: https://github.com/w2c/ispconfig3_roundcube
The account plugin is necessary for all other plugins, other than that you can activate each plugin seperately.
PS: The plugin you seek is the ispconfig3_autoreply plugin.
PPS: If you experience issues, read first the docu.