Unable to Centre Certain Text

I’m using the ‘amr shortcode any widget’ plugin that basically allows you to place any widget within the body of a post/page using shortcode which is pretty damn useful.

My problem is that the widget on my home page (MailChimp sign up widget) always stays to the left no matter how much I try to centre it in ‘Edit Page’.

Does anyone know how I could make the widget go in the centre of the page?

Thanks in advance ??