I'm afraid CSS Menu Writer isn't supported in CC 2015, you would have to upgrade to Design Extender, which includes menu writer.
At that point you can download the .zxp file from our site and install it directly using the DMXZone extension manager.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.