I have installed the extension both via Adobe Extension Manager and Adobe Exchange within Dreamweaver CC. Both show it as installed but I can't see any toolbar or find any reference to it in any menu so basically it's like it isn't installed at all. The same has happened with the Updater so looks like your plugins just don't work with CC.
All of the extensions that have a "Download ZXP" button on their product pages are surely working with DW CC. The 960 grid extension is located in the Insert PAnel, as shown in the tutorials. Make sure your Insert Panel is enabled (Window > Insert)