Hi,
Probably you got confused with geometry types.
Curves are geometry component that refers to Rhino Curve elements and it's native GH component.
If you want to reference polyline/spline form AC you need to use AC Connection elements. Such as Polyline or Spline. Those refer to AC Geometry.
I attach simple screen where GH elements are on the left and AC 2D geometry on right. Especially confusing might be point reference
Could you also change post topic to reflect problem?