Shoutbox

There are two options available to you;
The one you probably want is included in Messenger Discovery, you can set automated replies like this;

This can be downloaded at http://www.messengerdiscovery.com/

Another option is a quite sophisticated AI from The Blasphemer, it can respond with things and learn from your contacts with replies...a very good (yet unstable) AI engine...it can be downloaded from here.
You'll need a program that opens .rar files (like WinRAR that can be gotten on a free trial from www.winrar.com to open and extract all these files to your plugin folder (by default C:\Program Files\Messenger Plus! 3\Plugins)...