The Users Online Module displays a range of information regarding the site users.  User details are displayed in three categories (membership, People Online and Online Now) and the module can be set to display one, two or all of three of these categories.  

In order to proceed with the insertion of the User Online module, when logged in as an administrator, you have to follow this procedure:
Click
Add New Module and select the User Online module. The following dialog box will be displayed: