Zotero automatically installs the Word plugin into Word's Startup folder. If you've previously attempted to install the Word plugin manually, you may have accidentally changed the Startup folder to a location within the Zotero program files, which will prevent the plugin from working. If Zotero is telling you that your Startup folder is misconfigured, see Locating Your Word Startup Folder to learn how to reset the Startup folder to the default location.