Do the surfaces share the edges on the curve or are they separated by a gap? Or is there a face (hidden) continuing below the curve? It looks like that curve is not shared by both surfaces. Can you upload the model?
Round corner works by selecting edges that are shared between adjacent surfaces. Also there should only be two surfaces connecting to any of those edge.
From your description, it sounds like you are trying to work between two curves. Round Corner would work on a SINGLE curve, being the intersection of the surfaces.

firstlight! Kudos!
)
That hobby can keep you busy.)
