How to add a specific div element to the first post in a row?

Well… Of course it doesn’t work – $count is undefined and you never change it’s value.

You probably want to have there something like this:

<?php if ($loop) : while ($loop->have_posts()) : $loop->the_post(); ?>
    <li class="grid_4 portfolio-post">

        <?php if ($loop->current_post == 0) echo '<div class="newest">NEW</div>'; ?>

        <a href="https://wordpress.stackexchange.com/questions/138934/<?php the_permalink(); ?>" title="<?php the_title(); ?>">
            <header class="post-thumb">
                <?php the_post_thumbnail('thumbnail-portfolio'); ?>
            </header><!-- End header.post-thumb -->

            <aside>
                <h2><?php the_title(); ?></h2>
                <p><?php the_excerpt(); ?></p>
            </aside><!-- End aside -->
        </a>
        </li><!-- End li.grid_4 portfolio-post -->
        <?php endwhile; ?>
        <?php else : ?>
            <p>No portfolio items were found! I'm not sure what you're looking for.</p>
        <?php endif; ?>
        <?php wp_reset_query(); ?>
    </ul><!-- End ul.entrybox -->

WP_Query already has it’s own counter (current_post), so you don’t have to define your own.