I have an .igs solid file that I have loaded and want to use it to make a fixture plate. I have used a top surface of this solid as a plane to sketch a profile and extruded the sketch to size. Now I would like to use the IGS solid to remove the excess material from the extruded sketch to get a mated surface on the face of the extrusion. 
I’m having trouble finding the right command. 
I suspect I have made the extrusion a child of the imported solid parent but I’m not sure. 
When I try to hide the solid, the extrusion disappears as well with only a wireframe type geometry remaining. 
Please help me straighten this mess out

Thanks, Rick