To center the slideshow on your page, go to slideshow plugin's General Settings page and click the 'Custom Styles' tab. Then click 'Customize' behind the 'Dark' default stylesheet, since I see you're currently using that one. A custom style editor will pop up where you should make the following change:
margin: 0 auto; /* Add this line to center the slideshow */
Save the customized style, then go to your slideshow's admin page. In the style settings box select the newly created custom style. Your slideshow should now be centered.