That makes the question really easy.

In tpl_product_info_display.php, each section of output is contained between <!--bof xxxx--> and <!--eof xxxx--> comments; simply move the add to cart section below the attributes section, and they will fall into line. You may also need to change float or text-align in the stylesheet for proper arrangement.

It's the interaction of elements on the same line that is causing your present awkwardness.