Dreamweaver dropped support for the Extension Manager in CC 2015, so it won't install any extensions. The solution I suggest is to use a third party extension manager.
Like the one from DMXZone: http://www.dmxzone.com/go/22670/dmxzone-extension-manager-for-dreamweaver/
or Project 7: http://www.projectseven.com/products/ex-man/info.htm
It is free and will work with CC 2015