Visio does not support fitting a line of text to a curve. You can, however, create text that follows a curved path using WordArt in Word or PowerPoint. You can then copy and paste the WordArt into Visio.
Note: For information about how to create WordArt in Word or PowerPoint, see Insert WordArt.
To copy and paste WordArt into your Visio diagram:
Select the WordArt, and press Ctrl+C.
In the Visio diagram, press Ctrl+V.