https://blogs.adobe.com/dreamweaver/2015/08/installing-extensions-for-dreamweaver-cc-2015.html

The extension manager is no longer available to install extensions and Add-Ons for Creative Cloud version 2015 and later.

可见2013、2014、2015安装扩展的方法都是不一样的

IMPORTANT NOTICE:
Extension Manager obsolete in CC2015 ALL Creative Cloud users should move to the Creative Cloud Desktop application to install Add-ons.


2015 users MUST use the Creative Cloud Desktop application to install Add-ons.
It is still possible for earlier Creative Cloud versions (CC and CC2014) to use Extension Manager CC, but we STRONGLY RECOMMEND that you migrate to the new model.