I dont have photoshop but I think I read in one of the forums that you have to open both pics. Then load the selection from picture and pick the distance render. Like I said I dont have photoshop and I've never used distance render so I might be telling you wrong...but if I am I'm sure someone will correct me... as another option you could use the lasso tool and draw around the stuff you want to blur if it doesnt have to be exact.