You may use them but if your using HDMI you wont need them for your system as HDMI carries both audio and video.
And I agree with TC's post dont buy monster or any over priced HDMI cables you can get HDMI cables for much less $$ and there is NO difference in quality.
You would take one HDMI cable and hook that up to the DirecTV's box and run that into the back of the receiver/DVD player as they are the same unit. and then the second HDMI cable from the output of the Receiver/DVD player to the Samsung Plasma. In the menu of the receiver there should be an option to output through HDMI "turn HDMI On" or something like that.
Is the receiver separate in your system? Ok go HDMI from the DVD player to the receiver and HDMI from the receiver to the TV. How many HDMI inputs does the receiver have?
If it has two the you will need a third HDMI cable for the dishTV to the receiver otherwise you will need to use optical out from the DishTV to the receiver and component for the video.
Ok, you will need to use the DishTv directly to the TV using HDMI, Connect the optical audio cable from the dish to the receiver. HDMI out from the receiver to the Tv. This will get the 5.1 audio from the Dish to the receiver and alow you to use it for surround. You will have to select on the Samsung Plasma what HDMI input your using if you want to watch DVD movies or the Dish.