You have to make sure that you curve on the surface separates 2 (or more) areas. In other words, if it's a closed curve you are good, if it's 2 open curves intersecting each other at 2 points you are good, if it's a single open curve going from one side of the surface to the other cutting it in half then you are still good. I attached a picture showing examples of what curves would be right and what would be wrong.
I imagine that because you are making a car (that's you in that other thread, right?) and you are asking for a hole, that you have a closed curve probably in a circle. Well, in that case, after projecting the curve, select the surface, then click "Edit NURBS>Trim Tool". Then you will see you surface somewhat like the second picture bellow. Notice the diferent grid size, that's what's telling you what the different areas in the surface are (the open one didn't create one, see?). Now click on the area that you want to keep (in other words, outside the circle) and then hit enter. If you clise the wrong area just hit escape and then try again. If in my example I had wanted to delete the area outside the curves then what I would have clicked inside each of them and not outside. The yellow indicator is what tells you what will be kept, you can have as many as you want in each area that you want to keep (it doesn't make a difference if you have one or a hundred), just don't leave any inside the areas that you want to delete.
Now, this really sin't deleting anything, is more like hiding it. That's good, because then later on you can untrim the surface and get rid of the holes if you want. Also, if you are going to be creating a fillet inside that hole, you it's time to select an isoparm from that curve you can make it easier on you and chose "Trim Edge" instead of isoparm (hold right click, then move the mose south east).
Was that too much? I hope it helped. Let me know if that's not what you needed.
There can not be Good without Evil, so then it must be good to be Evil sometimes.