maybe you didn't insert the isoparms right? i gave it a try quick, and maybe this will help you out.

i made a nurbs plane, with just boundary edges, nothing runnign through it.
then i inserted all 4 of those isoparms at the same time (not sure if htat matters).
then i just pull the very outside row of CVs down and scaled it in a bit.

obviously it could be smoother, but its a start