It seems that you are having some issues with your Slingbox PRO HD, since you are not able to get the A/V signal.
If your Slingbox was properly set up for Internet Viewing, I recommend you to test first if the virtual remote control works fine. You only need to pull up the virtual remote control and press the power on button on it. Otherwise, make sure to reply back providing all the details about your home network and set top box in use. Make sure to include as much details as possible.