which part in the picture is your base? Meaning which was derived from your original sketch? How did you create the flange with the arc.
The sheet metal application in inventor is geared toward square bends(not meaning 90 degrees) from the look of the image above id say your part would need to be formed and not bent. I do not think inventor will flatten this because it would require deformation of your plate.
Typically if everything is drawn so that it is one piece and able to be flattened, you just hit the flat pattern button. I typically select the face of the flat pattern, right click and export as a dxf from there.
What is the part?