Show div based on custom meta value

Then you need a filter on the_content.

function add_conditional_div($content) {
    global $post;
    $meta_field = get_post_meta($post->ID, 'your-field-name', true);
    if (1 === $meta_field) {
        $content .= '<div>whatever</div>';
    }
    return $content;
}

The 1 value, of course, is whatever should match your meta_field.

http://codex.wordpress.org/Function_Reference/get_post_meta

http://codex.wordpress.org/Function_Reference/the_content

You could also edit your theme files directly or make a child theme. The latter is highly encouraged if you are editing a theme you didn’t create.