OK, this is all very basic, but you haven't told us what you have tried. Is the cable/satellite box turned on ? If so have you tried taking the video cables out of the Slingbox input and put them into a TV to see if they are actually carrying a video signal ?. If you are using an HDMI cable have you removed it to see if HDCP is stopping the video ?......
After some more work, I have discovered that my newly installed cable box, Cisco 4742HDC is apparently too new and is not shown on Slingbox's list of devices supported. How would I contact Slingbox to determine when they will add this new device to their list? I cannot find any way on the website other than paying $29.95 to call in. I guess I could go back and get an older model from Cox, my cable provider, but this new one is a whole home DVR. Thoughts Thx