It is free for the DMXZone extension manager. I've used it for over a year and haven't had any charges.
Once you close Dreamweaver you have to install again from the DMXZone extension manager. Does it give you any type of error message once you first close Dreamweaver and then install the extension with DMXZone extension manager?
Another option would be to install the extension from the command line:
https://helpx.adobe.com/dreamweaver/using/extensions.html#main-pars_header_1