The text editor opens automatically if you start editing a model in text format. You can do this by selecting the View>As Text command when working with the Sketch Editor or by using the File>New Model command. The Text Editor will also open whenever you open a model that was last saved from the Text Editor.
You can also start the Text Editor from the File>Edit File… command or by clicking on the Text Editor icon in the Workbench if you have set up your toolset to contain the icon. When you use the Text Editor icon, the File Selection dialog will open asking you for a file to edit. You can set options on the Text Editor to determine the default file type displayed when the File Selection dialog opens. Regardless of the options, however, you can edit any text file with any extension. If you attempt to open a file with a .vmf or .mdl extension, Vensim will attempt to load the file as a model.