Extruded cut in imported file

Hi,

I'm very new to this program. I started to use this program few days ago for part of my research project.

My goal is to make a negative mold of the condyle using Solidworks.

First, I imported  the part (3d shape of condyle) and draw a rectangle outside of the part. (Now the condyle is located inside the rectagle).

My question is when I tried to extruded cut the parts (which I imported) from the rectangle, it gives me a message saying "a plane, a planar face or an edge on which to sketch the feature cross-section, or an existing sketch to use for the feature"

So, I've tried to click on a plane and did the extruded cut but it gives me a empty 3d shape. I think I might have extruded cut the rectangle instead of the parts I imported.

Is it possible to extruded cut the files that I imported ?

SolidworksParts And Features