The first step to use Database Designer for MySQL is to create a new diagram.
To create a new diagram select the File | New menu item or press Ctrl-N. You can also use the New diagram button 
on the Standard toolbar.
A new diagram will have a default name like "Noname1". You can change the name of your diagram in the Diagram Properties dialog window.
See also:
Diagram: Diagram Properties
Interface User Guide: Ribbon