Some of the Draft Measure tools can be a little evasive to understand. Draft diameter is one of them. In order to get a diameter you must pre-select a curve.
The Draft Diameter Measure only requires the selection of an edge to add the dimensions.
Short Video Demonstrating Usage:
Step-By-Step Visual Selection
Select Draft Workbench
Step 1.) Select Draft Measure Tool from the Draft Toolbar
Step 2.) Click the “Select Edge” button from the Tasks Tab – Dimension Dialog
Step 3.) Click on the Circular Edge in the Document Window
Step 4.) Move the mouse to where you wish to position the ending arrow.
NOTE: The text will be oriented from the Starting Point to the Ending Point.
Bonus: to change the Dimension to a Radius, simply select False in the Data Tab under the property “Dimension”