Looks like this is a bug with the CC Extension Manager on Mac OS.
If you open DW, you should see that the extension is actually installed. I tested with eCart and got the same error, but opening DW, eCart was under the Insert -> WebAssist menu and was functional.
This issue is happening for other extensions as well, not just ours, there are a few threads about it on the Adobe forums, the Project 7 Forums, and DMX Zone forums.