cc 2017 dropped support for the extension manager.
The easiest solution is to use the free third party extension manager from DMXZone: https://www.dmxzone.com/go/22670/dmxzone-extension-manager-for-dreamweaver/
If you didn't want to register with them another alternative is to install the extension from the command line: https://helpx.adobe.com/extension-manager/using/command-line.html