Then Brightness is all you have. I've calibrated ACDs connected to Macs several times, and ever adjusted the Contrast. Even if it is available on the computer (I've never seen it), it would merely be a video card LUT adjustment, and as such should be avoided.
Look, I wasn't being flippant or dismissive. The ACDs are good monitors, so their contrast will be absolutely fine. Just adjust the Brightness to get a luminance that suits you, and you'll have a satisfactory result.