Just went through this with Muse-Themes widgets.
After you go through the instructions.
Here what Adobe fails to tell you, right now.
I kicked up a stink so this may become common knowledge.
Open Muse and at the bottom of the "Library" window NOT Widgets.
click on the add "Library" NOT a widget.
Look in the download folder that the .zxp file was downloaded to.
Look for a the same name as the .zpx file but with a .mulib ext and add it.
You now have a new Widget, confused so was I.
Hope this helps I chewed through two pair of Canadian wooly socks to figure this out.
Actually Extension Manager does not support Muse, for Muse extension, Extension Manager will list it into "Others". Please find your extension in download folder.
After acquiring where does extension goes, many apps have different downloading folders. Where can i find that particular folder, where that extension is downloaded?
I have already checked default folders for downloading in win8t. Please Guide.
Does the extension appears in folder C:\Users\<your name>\Downloads\? Extension Manager will copy "Others" extension to that forder.
If anyone needs the answer to this in 2017 ... after you install zipped widget with Extension Manager then find the same named file but with a .mulib extension. (probably in downloads same as zipped file). Then one you are sure it is there of to "Library" in Muse CC and click the icon on the bottom left of the Library to import Muse Library find the widget with mulib extension and when you click it it will install.