Arc Lengths

I am sure I am missing something easy, but I am all of sudden unable to get the arc length dimensions to work. I am trying to dimension vertical mullions on a curved curtain wall. Revit prompts me to pick the arc to dim, but will not pick any intersecting references to dim. Any help would be greatly appreciated.

Justin Hosler

Try this:

When it prompts you to pick the arc to dimension, first pick the endpoint of the curtain wall arc, & then you'll be able to pick & dimension the grid line points.

I knew something simple would do the trick, thanks for the reply.