You can try adding the following to the bottom of /includes/templates/YOURTEMPLATE/css/stylesheet.css:
Code:
#cartAdd br { display: none; }
#cartAdd input { float: left; }
That instructs the browser to not display the <br /> in the cartAdd box and to float all inputs (the quantity and add_to_cart button) to the left.
Without seeing your site, I can't help with the addThis button, but the solution would be similar (i.e. using float: left; in the CSS to float that element to line up with the add_to_cart button).
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