Hi David, are you trying to cut the surface with this curve here? :



A couple of different problems - one is that the surface kind of sags a ways away from the curve, if you look at it from the side a bit you can see this:



For a curve to cut a surface it's best for the curve to be either a 2D planar curve which will then project on to the surface by the perpendicular direction from its plane, or for the curve to be actually right on the surface instead of kind of a ways off of it like this currently is.

Then in addition to that, it looks like the ends of the trimming curve don't quite reach all the way to the edge of the surface (like Burr already mentioned, I just saw), which means that the curve will not fully divide the surface into different regions. You can see this if you zoom in to this area here:


So note there that when zooming in the curve is just a little bit shy of actually hitting the surface edge:



You want cutting curves to extend all the way to the surface's boundary to make sure that they will be able to cut the surface into distinct pieces. It's ok to actually make a cutting curve overshoot the boundary a bit, so you may want to do that just to make sure that it fully divides the surface.

I've attached an example 3DM model file that has a curve that should work to trim your surface now. I took your curve and flattened it in the Top view (just grab one corner grip on the edit frame that shows up when you select things and squish it down until you get the "flat" snap), so that it would be planar and project on to the surface, and then I turned on control points and grabbed the end 2 control points and pulled them up so they crossed the surface edge like this:



So with this version you can select the surface and run the Trim command, then select the cutting curve - that will then divide the surface into 2 regions, then you can select which region you want to have removed and right-click to finish the Trim command.

- Michael