You need to use the Deployment tool by Microsoft (first unlock your device for testing purposes).
See also: http://devproconnections.com/windows-10/windows-10-universal-windows-app-deployment-tool-available-now
Note: I had a lot of trouble to get the mxui.js loaded in a Windows 10 mobile / UAP app, because of MS script policy. In the end we decided just to create a web app, instead of a hybrid app.