How can I center my thumbnails?

If you have the pro version, update your slider and you can find the Center option at the Thumbnail control.

You should put this code inside you theme's head section:

<style>
.nextend-thumbnail-inner table.n2-ow{
margin:0 auto!important;
}
</style>

Or just simply paste it into one of your css files, without the style tags, and that will center your thumbnails:

.nextend-thumbnail-inner table.n2-ow{ 
margin:0 auto!important; 
}