As you'lle see in the posts above you've got 3 options.
1) Create the part as a flat part and use the bend/roll feature. (The best way but not always possible.
2)Change the model so that you've got a small edge that you can select as a reference for unbending. Proe will prompt with the following message" Select plane or edge to remain fixed while unbending/bending back." The system needs this edge or surface to determine the unbending direction.
3)Copy the outside surface and use the flatten quilt feature to unbend. This is very helpfull when you get those funny parts that just wont unbend. Just remember that this option will not take your y and k factor into account.