Are you metering off the *sky,* perchance? Or directly off sunlit concrete or shining waters, as you imply? (Are the dark images ones which have something really bright in the middle of the frame?)
Often when folks first get a polarizer, they of course want to photograph these things, you see.
Even with a polarizer, that will cause automatic exposure to want to treat bright sky (or whatever) as though it were a more ordinary subject. (ie, the camera will think it's a normal subject which it must reduce exposure to get you your photo.)
It's probably not anything you're doing 'wrong,' so much as things you're not doing.
There are several ways to effect some compensation for this kind of thing: probably the simplest, if you're using automation, is to point the camera a little away from the source of brightness, and use 'ae-L' to hold that reading while you frame the shot you'd like.
The camera's automation will be very clever, but it's not a mind-reader. It doesn't know that the very bright spots aren't supposed to look something like a human face under ordinary sun, for instance, (it only knows that there are bright areas and dark areas, in the case of 'matrix metering.' In the case of 'average' metering, it will simply take an overall reading biased toward the center.) Polarizers can help knock these effects down a notch, but they aren't that strong or precise: Concrete is a very reflective material, and omnidirectionally-so: unlike glass or reasonably-still water, it reflects at all kinds of angles, which polarizers simply can't cover all of.
(They can cut out the strongest: many of those which most directly-reflect the Sun, but not all. You still have a very high 'albedo' subject. It will *still* be brighter than your camera will assume your subject is supposed to look. Your eyes and brain are more 'automated' than the smartest camera. Part of photography, is actually learning to see the light more *simply* than you take for granted just looking at stuff.
)
Anyway, in short, this is part of why we have all kinds of manual and exposure-managing features, still.
For now, try pointing your camera somewhere without too much exceptionally-bright stuff in it, but under the same light, hold that reading, and reframe. You may start to get a sense of what's going on. (Also, if it's too dark, try using that +/- button to *give it more light.* I suspect with what you're describing, you'll want it to be +2 or +3 stops.)
Last edited by Ratmagiclady; 12-24-2009 at 02:58 PM.