To open an existing diagram file, select the File | Open command from the program menu. You can also use the shortcut Ctrl-O or press the File Open button in the Standard toolbar. The standard Open File dialog box will then be displayed. Browse to the diagram file you want to open and double click on it. Also, you can open a multiple diagram at once. For that purpose, please, select the multiple diagram files you need in the Open File dialog window and click on the Open button.
Please note, that Database Designer for MySQL diagram files have the .mdd extension.