YOu need the splines to be edited in this fashion. Only then will you be able to use sweep or lathe. If you want to do it otherwise, have a look at the "Push" modifier in the stack.
I'm probably approaching this wrong but just in case, wondering if there's a way to create a "plate" out of the spline shape you see here. I've tried sweep, lathe and extrude modifiers with no success.
No biggy Adrian, i would leave the spline for something else and draw all my attention at the push modifier because 3dsMax spline system is hard to swallow and is perhaps outdated. For a plate object have a try with the spherical object below everything and try to "Push-it" so you have a good extrusion to start with (not…