The theme is 1140px wide, not 960px. So if you adjust your settings to 1140px width then the slider will fill up the entire space of the container it’s in.
Also, the Genesis Responsive Slider images don’t center, they float left or right, but they don’t center. This isn’t caused by the theme or anything, that’s just the way the plugin itself is designed. You can change the width to 1140px to fill in the entire space which will make it appear as centered.
You should also download and run a plugin called Regenerate Thumbnails anytime you change or add your image sizes. This will adjust everything instantly for you so you don’t have to wait for the images to resize.
I create awesome sites for awesome people! Contact me if interested – firstname.lastname@example.org