You need to configure the outgoign smtp mail server for Confluence first. If you did not configure the outgoing mail server, this app does not work. You can find the documenation how to configure the mail server here.
Installation & Licensing
Log in to your Confluence instance as an administrator.
Click the admin dropdown and choose Atlassian Marketplace. The 'Find new apps' screen will be displayed.
Use the search function to locate 'E-Mail this Page'. The correct app version will be shown in the search results.
To start the app click on 'Install' and the app starts downloading.
Configure custom CSS for the e-mails
Navigate to the Universal Plugin Manager and select 'E-Mail this Page'
Click on the magnifying glass icon to select the Confluence groups that are permitted to send e-mails (alternative: you can enter the names and split them with a comma)