Please see the link below for details
https://marketplace.concretecms.com/marketplace/how_to_install_add_ons_and_themes_/
Firstly please register for a http://groupdocs.com account. Once you upload your document you'll be able to find the Document ID by reviewing the embedded document link. This can be found by viewing a document in the main GroupDocs Viewer, and clicking the 'Embed' button.
The document ID is highlighted in the screenshot below: