I don't have a K-X but have the K200D and K20D and started off using the AF280T flash (which is great by the way!).. I may not be able to explain all the technical details with regards to using a flash but here's what I've learned from just plain ol' taking lots of photos:
Keep that flash in either Auto Green or Red.. Green gives a more powerful flash and I find that's what I tend to keep it on, especially because I always bounce the flash off a ceiling or wall so I need more power to produce enough light (ie. the light has further to travel as it needs to go from the flash, reflect off a large surface, and then bounce back to my subject).
Shoot in manual mode so you can independently adjust each setting to obtain proper exposure.. of course, you need to understand how each setting affects exposure.
ISO will affect how sensitive your camera is to the flash as it is fired. Higher ISO = brighter picture.
Shutter speed affects how much ambient light your camera will capture. Keep it 1/180 or slower as the flash will not fire above that speed. Higher speeds = less ambient light. You may have seen club photos where only the people close to the camera were exposed while the background behind them is pitch black.. that's what happens with flash + high shutter speed and imho those pictures don't look very nice as you lose the ambiance of the location. The opposite to that effect is when you "drag the shutter", or use a very slow shutter speed (around 1/15 or slower I think) to purposely create light trails from the ambient light (the initial flash will freeze your subject in the image so it'll look sharp but other sources of light will be streaked on the image if you move your camera during exposure).
Aperature will affect both (as well as depth of field). The larger your aperature (small f number.. ie. f1.4 or 2.8), the more light your lens will let into the sensor. When shooting flash I prefer to keep it between f5.6 to 8.0 as I find it's easier to balance the different light sources with those aperatures.
I don't pay TOO much attention to the exposure meter when I have the AF280T mounted because your meter gives you the reading WITHOUT taking into account the effect of the flash. That's why when you were doing test shots and trying to get it exposed properly with the meter, you were getting OVER EXPOSURE once the flash fired.
These days I have an AF540fgz flash that's much more automated so I tend to worry less about getting proper exposure.
Hopefully what I've written is understandable.. and if anything is incorrect, another forum member who's more knowledgeable than I can correct it.