From your description, it seems as if your monitor is hooked directly up to the satellite network's receiver box. That means that most likely the box is acting as a TV, providing the tuner necessary to locate channels to display on the monitor.
In standard TV sets, the TV is the tuner and it contains the closed captioning technology. Since you have no "TV" per se, the trick is to figure out where the tuning is taking place and locate the closed captioning.
The early plasma monitors did not have closed captioning technology. However, as the boxes have become more sophisticated, many also contain captioning technology - just as many also have built-in tuners.
So captioning technology may be available in either the Dish box or the monitor. By law (Television Decoder Circuitry Act of 1990) it MUST be available in whatever product is providing the tuning. However, not all tuners are compliant with the law.
We recommend that you look through both the menus of the satellite box and the plasma screen for "screen" options. If you are still unable to find any options for closed captioning, our recommendation is that you contact the satellite service provider and politely inform them that you are unable to find the captioning on their tuner as required by law and that unless they are able to help you findit, you will require a replacement with a box that does have closed captioning capabilities.