Was Dreamweaver Open when you installed them?
If so, you will need to restart Dreamweaver.
I have seen instances where the Extension manager install was corrupted. If you accessed the Extension manager by going to the Start menu, you would see one list of extensions, if you accessed it in Dreamweaver through Commands -> Manage Extensions you would see a different list.
access the extension manager directly through Dreamweaver, once open, close Dreamweaver and install the extensions manually by selecting the Install Button in the Extension manager.