Thick cut on curved surface - not working for me

Hi everyone,

In the attached file i want to cut the split spline i made and remove that surface, but it wont let me cut the curved ones. Someone tried helping me and I so that it is possible to do, but he has the 2012 version and i use the 2011 student. I did see that he used untrim surface after the offset, i dont know why or if it make a difference.

What can i do?

SolidworksSurfacing