It seems that you are having some issues in order to control the Pace 7121 set top box since it is not included on the supported devices list.
The main issue is that we do not have the IR code that will allow you to control this specific device. With this in mind, I recommend you to run the A/V source configuration and test all the available Pace IR codes. If none of the codes seem to work, I recommend you to check this thread, provided by alanrichey42:
I also recommend you to check the following link in order to determine if that computer meets the requirements for the Slingbox SOLO:
Hope this helps.