You can install Unstoppable via the Atlassian Marketplace. Administrators can find and install apps from Marketplace in the apps administration pages as follows: 

Administration Guide for Confluence 

As a system administrator, navigate to the application's administration console and click "Add-ons".

Find the Add-on 

Under Manage Add-on Section, click the "Find new add-ons" link.

Search for Unstoppable in the Marketplace 

Type "Unstoppable"  in the search box and hit "Enter"Click "Free Trial".

See Also:

After we have the plugin installed, next is to enable Unstoppable post installation. This can be done with several methods. The first method is to enable the plugin: Post Installation Steps for Unstoppable for Confluence 

Next is to validate that Unstoppable is installed: How to determine if Unstoppable is installed in Confluence