I don't know about the mac version, but it should be the same as the windows version. With the slider bottomed out, what does the number in the box next to it say? It should range from -100 to 100, with 0 being no adjustment & -100 being no color at all.
Now, the sliders remember the last used setting, so you can't just go by it's current position. If you have the show preview and/or apply to image boxes checked, the effect of the current settings should be visible in the after panel and/or on the image itself. If you have both of those turned off, try hitting the ok button and see if it changes the image.
If the slider won't go below 0, then you should report it as a bug in the bug section of this forum.