I understand that you are having some issues with your Slingbox PRO-HD since you cannot control the attached set top box.
First, I recommend you to check if your set top box is included on the supported devices list. This is the link to check:
Finding your video source
If you are not able to locate it on this list, I recommend you to test all the available Thomson IR codes. If none of the IR codes allows you to control this device, I encourage you to check the following thread provided by alanrichey42: