I was able to center it on my test forum by adding <center> tags to the beginning and end of the code in the Main Header/Footer CSS.
Go to plugins and select build > iSlider > Components > Main Header/Footer at the bottom of the page and wrap it in <center> tags as below.
<center><div class="slide-wrapper theme-default">
<div id="islider" class="nivoSlider">
{foreach $[plugin.settings.slide]}
{if !$[plugin.settings.slide.hide]}
{if $[plugin.settings.slide.slide_link]}
<a href="$[plugin.settings.slide.slide_link]">
{/if}
<img src="$[plugin.settings.slide.slide_image]" style="height:$[plugin.settings.slide_height]px; width: $[plugin.settings.slide_width]px;" data-transition="$[plugin.settings.slide.slide_effect]" title="$[plugin.settings.slide.slide_description]" alt="" />
{if $[plugin.settings.slide.slide_link]}
</a>
{/if}
{/if}
{/foreach}
</div>
</div>
<script type="text/javascript">
$(window).load(function() {
$('#islider').nivoSlider();
$(".nivoSlider").css({ width: proboards.plugin.get('islider').settings.slide_width+"px" });
});
</script><center>