The HDCP error you are getting is the result of a layer of copy protection coming from the cable box (called High Bandwidth Digital Copy Protection, or HDCP). It sounds like you're running an HDMI cable from your cable box to the TV, and running component cables (red, blue, green, red, white) from the cable box to the Slingbox. The HDCP triggers because your cable box is outputting its signal to two different places, and it shuts off the signal to a device it thinks is stealing its content, even though that's not what a Slingbox does. There are a couple ways around this:
1) Disconnect the HDMI cable altogether and run a second set of component cables from the Slingbox out to the TV (don't worry, you still get your HD signal!).
2) If you wish to keep using your TV with HDMI, you can use an HDMI adapter to strip the content protection. Here is a little information on that:
Hope this helps!