With just the radius and a distance, im assuming its a tangent curve from the last segment. The distance could be either distance along the curve (arc) or the chord length (chord).
In the traverse dialog choose 'Tangent Curve' then set the arc/chord, radius and direction parameters. 'Add' should then be enabled.
If 'Northerly' actually means something, it could be the chord direction when entering as a curve, but i wouldn't think so in this case.
I hope you don't have any true 'spiraled curves' either, as they cant be entered through the UI.