I quote: "The extension manager is no longer available to install extensions and Add-Ons for Creative Cloud version 2015 and later. This article discusses how to install extensions and Add Ons for Dreamweaver CC 2015 and later versions." see
But there is >>> http://www.dmxzone.com/go/22670/dmxzone-extension-manager-for-dreamweaver >>> Get DMX Zone's Extension Manager and here I quote too: "DMXzone Extension Manager for Dreamweaver is a stand-alone application from a new generation that will make your life easier. In a blink of an eye you can install, update and manage your extensions and templates for any Dreamweaver version above Dreamweaver 8."