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You need to be a Confluence Administrator in order to install Embedder.
First, login as the Administrator to your Confluence instance and select "Add-ons" 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 add-ons" 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 add-on being installed and activated.
A confirmation dialog will tell you that the installation was successful.