This is quite easy to do when you know the trick. Just add a couple of new styles to your style sheet:
.widget_counter_widget { text-align: center; }
.odometer-widget { display: inline-block; }
First one just centers content in the counter widget container, the second and most important one will allow the actual counter to be effected by text alignment.
By the way a new version will be out in or week or two that has had a major rewrite, has lots of new features and is easier to customise. The info above is for the current (at the time of writing this) 1.0 release and may not work on the next release…
Version 1.4 now features alignment options for both widgets and shortcode counters, no more need for the above.
