You can open an existing mix file and load it into a new Mix Design Form by clicking with the mouse on the Open item of the File menu.
Shortcuts
Mouse:Click on the toolbar
button.
Keys:CTRL+O
The names of the existing mix files will be displayed in a File Open Dialog Box.
You can open several mix files at once. Use the Window menu to switch between the open mix files. See Window 1, 2, ... command.
You can create new mix files with the New File command.