For the following steps you need Confluence Site Administrator permissions.
The add-on is available at the Atlassian Marketplace. To subscribe the add-on, log into your Confluence as a site administrator and navigate to the "Find New Add-ons" tab in the global settings. There you simply search for "SubSpace Navigation for Confluence". For more details about the Universal Plugin Manager please refer to the documentation provided by Atlassian.
The add-on needs a valid license, otherwise features might not work. Valid licenses are subscriptions or 30-day trials and are available at the Atlassian Marketplace. The license model is the same that Atlassian uses for its products – for further information see the Atlassian documentation.
Image 1: Add-on Manager (click to enlarge)
To obtain a license and apply it for the add-on, please carry out the following instructions, (also see the screenshot above):