Not sure what you're asking. I'm sure many people know how. Perhaps you have a more specific question.
Note that you can't dimension to the edge of a curved surface.
I can't seem to select the individual balustrade member, but you might have hit the nail on the head regarding dimension to a curved surface. Didn't think about that. I'll try that and see if that is the problem.
Are you talking about a curved rail, or a straight rail with round elements? If the 2nd one, split the rail profile into 4 quadrants and hover over the end of the rail (highlighted – see pic) while dimensioning. Hit a tab key until you highlight desired point. Dimension tool will sense the end points of the profile quadrants.