The Great Glowy Box is central to any game of Munchkins, and how you deal with it should be carefully considered.
It is generally the recommendation of the author that, if at all possible, you should use a real television in your games. Whenever the Munchkins manage to get a glimpse of the GGB, flick on your television and give them a few minutes.
However, you may find that your local programming does not lend itself to humorous interpretations, especially if your gaming group is not accustomed to a style of play that requires on-the-fly improvisation of this nature. In that case, you may wish to simply inform the players the sorts of things going on on the television. If you do employ this method, use it for all its worth -- if you narrate properly, you can describe a real-life television show from the point of view of a Munchkin, which can be quite humorous when the players figure out what you're actually describing. Don't concentrate on the narrative line or plot of the show -- Munchkins aren't capable of understanding that. Describe things visually, with as much detail as possible while retaining a sense of continuous action.
In general, though, give the real thing a try. It will contain a wealth of details you could not hope to include in any kind of verbal description.
Remember that Munchkins can not understand any human language. It may be that your players will have an easier time focussing on the Munchkin sorts of details if you mute the television or turn it to a station that plays in a language you're not familiar with. However, don't take this restriction too seriously. If the players slyly use some language as inspiration for their antics, that's okay -- it's a humor game, for god's sake! Munchkins might, for example, imitate some of the sounds from the GGB without understanding their meaning. Munchkins mouthing inappropriate phrases from a TV show sounds like a fine idea to the author.
Avoid shows that contain lots of people talking and little else. There's nothing to work with there. Similarly, avoid shows that contain sets or situations far removed from what can be found in a typical suburban home. The idea is for Munchkins to misapply the values they're learning from the GGB to their day-to-day life -- there's little to relate to if you're watching Star Trek.
Ideal Munchkin shows contain lots of action, low production values, and lots of modern-day paraphenalia. Some good example shows are: The World's Funniest Home Videos, Buffy the Vampire Slayer, Southpark, Iron Chef, and almost any commercial.
It may take your players a while to get the hang of how to relate TV programming to a Munchkin game. If they're having some problems, you can suggest things to them, or have an NPC Munchkin do some basic interpretation.
In general, you can look at any human on the GGB as either a human or a Munchkin, whichever is funnier. So, taking a show like, say, Walker, Texas Ranger, you could interpret Chuck Norris' character as a human, in which case the Munchkins might think that the Arch-Devil is a red-headed human with a beard, and try desperately to kill the family teen when he starts experimenting with a goatee, or as a Munchkin, in which case the Munchkins might all try to emulate Walker's actions.
Any mundane item which is prominently featured on a show might be a sign that that item is blessed by God, and the only course of action for the Munchkins is to steal it and set up a shrine to worship it. Or that it's being pointed out as the arch-enemy of Munchkin-kind, and it is their duty to destroy these things.
Or they may go looking for things which are, for all intents and purposes, mythical -- If they see the intro to Northern Exposure, they may decide to quest for a Moose.
Anything which causes harm to cats, dogs, or the like (classic Warner Brothers cartoons or their immitators are great for this) can be interpreted as a formula of actions to follow.
Finally, a show which features lots of explosions and the like may incite rampant pyromania, which is inherently amusing.
If you're having trouble digging up good adventure seeds from the GGB, try recording some particularly appropriate show and using that to go off of.
Place appropriate props around the house. Children can be a lifesaver here -- there are lots of dolls and toys (models of the Enterprise, or action figures) which might figure into popular shows. Magazines also often feature TV stars on their covers.
Stick to short bursts of TV (you can have the humans or pets break up any long sessions) to prevent your players from getting interested in the GGB for its own sake. The GGB is a device to provide material for improvisation, not something that should interrupt the flow of your game, and some humans have as much difficult tearing their attention for the GGB as do the Munchkins.
If nothing's happening with a GGB session, chase the PC's away from it in-game, and let them come back to it later.
If there's still nothing happening, you can throw the PC's into a panic with a power outage -- the Great Glowy Box has gone silent! There must be a heretic in our midst! Etc.