When you download the Zotero Desktop Application, the plugins are downloaded too. You will need to go into Zotero in order to enable them. The current plugin works with GoogleDocs, Microsoft Word, and LibreOffice. The plugin allows you to add citations and create bibliographies. Your Zotero desktop application must be open when you are using the plugin.
You need to have the Zotero Chrome Connector installed in order to see Zotero in Google Docs.
While this plugin (add-on) should install automatically when you download the Zotero App, you might need to do extra troubleshooting in order to activate it.