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".
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