You need to be a Confluence Administrator in order to install Embedder.
First, login as the Administrator to your Confluence instance and select "Apps" from under Confluence Administration. Confluence Administration menu is available in the top right corner on click of the 'Cog Wheel' icon.
In the "Find new apps" page, search for "Embedder for Confluence".
Click on the "Free trial" link on the right side of the corresponding listing
This will result in the Embedder for Confluence app being installed and activated.
A confirmation dialog will tell you that the installation was successful.