How do I add the web importer to the new Microsoft Edge web browser?
The MasterCook.com web importer is a bookmarklet. The biggest complaint people have who use Edge (including most who don't even use MasterCook) is its lack of bookmarklet support at this time. Hopefully Microsoft will continue to improve Edge and eventually add drag/drop support for bookmarklets. Windows is constantly pushing out updates for Windows and Edge, so it will likely eventually appear.
In the meantime you will need to add the bookmarklet to the Chrome or Firefox web browsers. In Edge you can import bookmarklets from Chrome or Firefox which will bring it into the Edge web browser.