It seems that you are having some issues trying to control the Scientific Atlanta box attached to your Slingbox PRO-HD.
Considering the provided information, I encourage you to run the A/V source configuration one more time and use the "Cisco 4250HDC" or "Cisco 4250SDC" as possible options. If none of them work, I encourage you to test all the available Scientific Atlanta IR codes in order to determine which one will allow you to control this specific box. You can also check this link in order to get further details:
Hope this helps,