The input is for peripherals like an X Box. What you are doing is using your display as a tv monitor. You cannot use the computer at the same time if the display is being used as a monitor. Connect the cable connection to the tv tuner antenna connection skipping the set top box.

Verizon has directions on connecting a computer   www.verizon.com/.../computer.htm

Thank for the clarification Mary. I got the slingbox for a gift, so I will use it to stream my cable tv cahnnels to my desktop. Regards.