Thanks for contacting us. You mentioned that you are able to watch TV on your own computer, but not your Parents, Correct? You might want to check out a article below that may be helpful to you:
What browsers and systems are supported by Watch on Slingbox.com?
An important thing to see in this article is that if you have Mac OS computers, then they must have Intel processors to use Watch on Slingbox.com or Setup on Slingbox.com.
Hope this information helps!
The Slingbox Moderation Team