If the virtual remote isn't working, there are a few things you can try. You will want to make sure the IR Emitter is placed in front of the sensor on your cable box, make sure that the cable box is in IR (Infrared) Mode, and make sure that you are using the correct virtual remote for your setup. The articles below should help you out further.
- Slingbox.com - Set-top box not responding to remote commands
- Slingbox.com - I got a new set-top box, and I can't control my TV now
- Slingbox.com - Advanced IR Emitter or Onscreen Remote Control Troubleshooting
Hope this helps!