If your rubber plug was the type that uses a wing nut on a screw to compress the rubber and the wingnut was on the screw far enough so that some threads were still visible, go to home depot and by a threaded coupler and a 3ft threaded rod that matches that thread size. Get an extra nut of the same thread size. Spin on the nut up the rod an inch, spin on the threaded coupler about half way on. spin the nut down to the coupler and thighten the two together so they don't spin on the rod. Take this device and slide it down the pipe and with lost of trial and error thread it onto the threads on the rubber plug. Then if your lucky you can pull it out. Let us know how you got it out.