It seems that you are having some issues with your Slingbox, since you cannot get the A/V signal.
Most of the times, this issue is related to connections you use on the network. If you are using the HDMI cable, you will need to remove it, since it is not compatible with the Slingbox (no matter which Slingbox model you have). The following link will provide you with the explanation and a possible workaround.
If you are not using the HDMI cable, I recommend you to check the resolution on the cable box. If you are able to get the menu but you get nothing else, there's a good chance that the root cause is the resolution on the cable box. In this case, you'll need to downgrade the resolution on your set-top box.