(SMART widget is used only as a sample for this article, this method can be applied for all widgets)
After you downloaded the widget, unzip the file, and you will have a folder with usually 3 files in it.
Go back to Muse, open the library panel from The top menu Windows > Library.
When the library panel is open, select the import button at the bottom of the Library panel. A window opens and you can select the .mulib file.
After you imported the widget, it adds a new folder to your library panel called after the widget's name. However is most cases, the new folder name is called QooQee Widgets.
Now you can drag and drop the widget from that folder, in Library panel, into the project.