How to communicate with arduino uno when arduino motor shield connected to arduino uno?
Communicate with what? A PC? The usb serial port should still work fine, and anything connected to the RX/tx pins (D0 and  D1)
If your concern is that there is no longer a socket on the top of this shield that you can plug wires into, well you can solder on some wires to the top of the board, or you could get a screw shield board to go between the arduino and motor shield which would make all the pins available. I have also use small gauage wire wrapped around the shield pins and then just insert the shield onto the arduino making kind of a wire wrap connection between the boards.