Is the cone solid all the way around? There needs to be a rip (split) along the cone somewhere or there is no way to unravel it. Use the rip command to create a rip along edge. I have done it along the curve that is on the intersction of a dtaum plane and the surface of the cone. The cone cannot go all the way to the top. This is not theoretically possible for a sheetmetal part.
the part shown in the image is not a pure bending component. hence you are not able to unbend or flatten the component
if you look carefully, the line of bend is not a straight line. its a hyperbolic curve (correct me if i am wrong, but definitely not a straight line). so any bend made on line other than a straight line would not be a pure bending operation. it would be forming/flanging operation and hence Proe will not help you unbend it.
take any plane passing through the axis of the cone and intersect it with the surface of the cone to create a curve. this would be a straight line. any plane other than that would result in a some sort of a spline.
I used a simple unbend. I used the edge of the rip in the cone as my fixed edge. I used a regular rip, for my sketching plane I created a datum that was normal to the cone, through the curve, and I think perpendicular to a plane. I skecthed a straight line (use edge, pick the curve) as my rip. I could re-create the parts and send you them, but I don't know where you are.