- draw the full circle of the lower curve and the upper curve ..
- loft the two circles with no guidelines or pathes ..
- delete the upper face for the last created solid .. that will give u a cone ..
- slice the cone using the center rectangle part of ur top 3d polyline as a slice plan ..keep the lower part ..
- slice it again using the two side guidelines as a plan..
this will eventually give u how the curve should look like if the 3d polyline is planner ..
- draw a closed polyline connecting the center rectangle and the two mirrored curved we produced earlier ..
loft the last polyline with the original lower polyline using the six guidecurves u have .. ( don't forget to extrend the straight guidelines to reach the new polyline)
- this will give u the 3d solid u need .. all what's left is to go to front vies and ucs .. thin trim the unwanted solids with slice or subtract using a new shape (that will b better cause slice may split some faces)..
am sorry i couldn't attach the 3d solid i already modeled at work .. couldn't log-in from there ..
your spline has 3d coordinates(not flat). The polyline is flat so there is nothing wrong with it. Whenever you try to loft using curved surfaces/closed lines, you will only get a surface. Try to flatten the top spline and you will see that you can create a solid.