You first need to download the plugin. The latest version available can be found here.
Before installing the plugin, ask your SightCall Administrator to make sure you do not have a plugin installed (Admin Plugin). In most cases if that's the case you should not be able to install the plugin on your own.
Installing the plugin is really easy. Once you downloaded the installer, all you need to do is to execute it and follow the steps.
Windows (RTCCplugin.msi):
The plugin is now installed for all plugin-compatible browsers.
Mac OS X (RTCCplugin.pkg):
The plugin is now installed for all plugin-compatible browsers.