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Dark is subjective, so just so we're talking about the same thing, the viewfinder brightness should be similar to the DA-L 18-55mm lens that came with your camera when that lens is close to 55mm. They'll vary a little and can't be set to the same focal length, so you can't see exactly the same view, but shouldn't be dramatically different.
The lens has an aperture that opens and closes to let in different amounts of light. When it is correctly attached to the camera, that aperture is always fully opened. Your F80-200 has an aperture ring that can open and close this aperture manually. Normally, you should set the lens's aperture ring to its A position and leave it there. On the K-x, the lens will work better that way.
As suggested above, make sure the lens is correctly mounted until it locks. It should feel exactly like mounting your kit lens. If you mount the kit lens, you can see the aperture blades move from fully closed to fully open as the lens is rotated into locking position. That's what should be happening with the F80-200. The blades are a little deeper in the lens but you can see them.
If that doesn't help, the only thing I can think of is a mechanical problem with the aperture lever. That is the protruding part on the lens mount that allows the camera to move the aperture blades. It could be bent, so the camera's control arm doesn't make contact with it. Or the spring could be broken or missing. With the lens off the camera, you should be able to flick this spring-loaded lever and make the blades snap open, then spring closed. You can also turn the aperture ring from A to the opposite end (I think it says 4.7 on this lens) and the blades should open up also. The aperture lever should look a lot like the one on your kit lens, sticking straight back.
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