How do you run monitors on your computers? Like, do (they) make a receiver to convert the red/black cables to a 3.5mm jack?
Virtualrain's idea is what I do when I need the highest quality sound, as my interface (Presonus Firepod) has balanced outputs. This helps to eliminate audio issues such as hum or having your cables pick up radio signals. I've been fighting a bit with using the regular 1/8" stereo output on my Mac. I get some odd audio artifacts every now and then, but these are eliminated when I use the balanced outputs on my Firepod.
In a pinch, however, I've made an adapter cable that connects my Mac's 1/8" output jack to two pseudo-balanced 1/4" TRS cables. If you are interested in building your own, see wiring diagram "2.1" on the top of page 2 in this document:
http://www.jensentransformers.com/an/an003.pdf
Music stores should also have proper adapter cables on hand (and probably of higher quality than the quick one I made!).
That solution does reduce radio frequency interference, but I still get weird artifacts (especially humming) at different times, usually depending on what else is plugged into nearby electrical outlets. My house's electrical system obviously has some minor grounding issues. My cell phone charger in particular is pretty bad if I plug it into a certain electrical outlet... I hear tons of hum with that thing plugged into a nearby outlet and my speakers plugged into my Mac. If I was just using my Mac's audio out jack (instead of my Presonus interface) and wanted to eliminate all the hum, I'd need to pick up a couple of audio transformers and wire them in (or use special direct input (DI) boxes that you can buy for this purpose).
In short, if you are using your Mac's output and you are getting hum or other audio artifacts (I've even picked up AM radio signals with a bad cable!), you might need to get an adapter cable or some transformers / DI boxes, or on an audio interface. These things can be really really difficult to diagnose, unfortunately.
Hopefully you won't have any of these issues with a simple adapter cable and can completely ignore most of this post!
(And now I've gone way off topic.. heh... umm, so, yeah... the KRKs are awesome speakers!)