I have this problem with all extensions I have tried to install; not just Google Maps. Extension Manager CC version I have is 184.108.40.2067 which should be the latest. I'm trying to move from CS6 to CC but can't until/unless I can get my extensions installed into CC. So far, no luck.
Added information: When I started Dreamweaver the extension when through its install process and seems to have installed into Dreamweaver CC. However, when I open the Extension Manager CC application the extension does NOT show up there. So it seems like the error is with Extension Manager and not the application itself. I did note that this only appears to work with one extension at a time. If it tried to install a second extension BEFORE going through the install process in Dreamweaver CC the first installed but the second did not. Hope that helps.