With Liferay 5.1, we introduced a new Chat Portlet.  Here are instructions on how to add it

Start by getting to the "Plugins Installer".  You'll see that the Chat Portlet is one of the options.

Select the latest version of the Chat Portlet and click "Install".

The Chat Portlet will start to install and you will see a message that it has installed successfully.

You'll see that your Tomcat console has the Chat Portlet install logs.

Once you goto another page, you'll see the portlet at the bottom.