MindMup is a general purpose mind mapping tool, and argument mapping is just one use case that it supports. the
argument.mindmup.com site is a shortcut that automatically creates a new map in the argument visualization mode; the main web site creates a blank map without any specific mode. you can switch to arg viz from the view menu (View -> Start argument Visualization) or you can create a new map from that screen in arg viz mode from the File -> New menu.