The problem is in your theme, not the plugins, specifically that it isn't loading the jQuery plugin "mobileMenu". In chrome the error is being reported on line 8 of /wp-content/themes/max-magazine/js/custom.js:
Uncaught TypeError: Object [object Object] has no method 'mobileMenu'
jQuery('#nav .menu').mobileMenu({
defaultText: 'Navigate to...',
className: 'select-menu',
subMenuDash: ' –'
});Comment this out and (assuming there aren't further errors) you should see the add to cart button.