You use the Name and Notes function to give the selected tile a name. A text field appears on the drawing page, with a pointer to the instance concerned. The name proposed by the Structure Modeler is name of the underlying system organizational unit.
Use the standard Visio functions to edit the name.

The text is connected to the instance to which it refers with an arrow. You can place the text field and the related pointer wherever you wish. Select the text field and hold down the left mouse key. You can then move the text field with the cursor.