Note: The UML Model Explorer is not available in Visio 2013 and newer versions. For information on UML diagrams in Visio 2013 and newer versions, see UML diagrams in Visio.
Drag a Note shape from any UML stencil onto the drawing page and place it near the elements you want to add a note for.
Drag the control handle in the middle of the note to create a pointer. If you want to associate the note with a specific element, glue the control handle on the pointer to a connection point on the other element.
Double-click the Note shape. Type a name for the note if you want. Under Documentation, type the text for the note, and then click OK.