Open Source Document Management System | OpenKM

Is there any one know how to install the forum plugins?
It is in opemkm's demo site.

Take a look on user guide http://wiki.openkm.com/index.php/User_Guide all features under Professional extension only comes with the customers installation are not present on community edition. That it's not part of the core are extensions developed as is explained in http://wiki.openkm.com/index.php/Extension_Guide

You can not install it, are not present on OpenKM community. You can develop your own extensions or become a customer.