Next & Previous (Pagination in this case) Not Appearing

I am using a custom page template for my portfolio. The code is calling the correct number of posts per page but for some reason the pagination links won’t show up :-S

My query

<?php 
    $loop = new WP_Query(array('post_type' => 'portfolio', 'posts_per_page' => 2)); 
?>
<?php while ( $loop->have_posts() ) : $loop->the_post(); ?>
<?php   
    $custom = get_post_custom($post->ID);
    $screenshot_url = $custom["screenshot_url"][0];
    $website_url = $custom["website_url"][0];
?>

The entire markup

<?php
/*
Template Name: Portfolio
*/
?>

<?php get_header(); ?>

<div id="full_container">
<div id="portfolio_content">
<div id="portfolio_wrap">
<div id="content">

    <?php 
        $loop = new WP_Query(array('post_type' => 'portfolio', 'posts_per_page' => 2)); 
    ?>
    <?php while ( $loop->have_posts() ) : $loop->the_post(); ?>
    <?php   
        $custom = get_post_custom($post->ID);
        $screenshot_url = $custom["screenshot_url"][0];
        $website_url = $custom["website_url"][0];
    ?>


<a href="<?php the_permalink() ?>">

<span class="img">
<?php if ( has_post_thumbnail() ) { the_post_thumbnail( 'thmb-portfolio' ); } ?>

<span class="under">
<!-- Excerpt title -->
<span class="title"><?php the_title(); ?></span>

<!-- Excerpt description -->
<span class="desc">
    <?php my_excerpt('short'); ?>
</span>
</span>
</span>
</a>


        <?php endwhile; ?>  

<!-- Next/Previous Posts -->
<?php if (function_exists("pagination")) {
    pagination($additional_loop->max_num_pages);
} ?>

</div>
</div>
</div>
</div>
</div>

<?php get_footer(); ?>

Solutions Collecting From Web of "Next & Previous (Pagination in this case) Not Appearing"

<?php
/*
Template Name: Portfolio
*/
?>

<?php get_header(); ?>

<div id="full_container">
<div id="portfolio_content">
<div id="portfolio_wrap">
<div id="content">

    <?php 
        $loop = new WP_Query(array('post_type' => 'portfolio', 'posts_per_page' => 2)); 
    ?>
    <?php while ( $loop->have_posts() ) : $loop->the_post(); ?>
    <?php   
        $custom = get_post_custom($post->ID);
        $screenshot_url = $custom["screenshot_url"][0];
        $website_url = $custom["website_url"][0];
    ?>


<a href="<?php the_permalink() ?>">

<span class="img">
<?php if ( has_post_thumbnail() ) { the_post_thumbnail( 'thmb-portfolio' ); } ?>

<span class="under">
<!-- Excerpt title -->
<span class="title"><?php the_title(); ?></span>

<!-- Excerpt description -->
<span class="desc">
    <?php my_excerpt('short'); ?>
</span>
</span>
</span>
</a>
<?php previous_posts_link('Previous'); ?> / <?php next_posts_link('Next') ?>

        <?php endwhile; ?>  

<?php if (function_exists("pagination")) {
    pagination($additional_loop->max_num_pages);
} ?>

</div>
</div>
</div>
</div>
</div>

<?php get_footer(); ?>

I don’t see the variable $additional_loop being set anywhere in your code. If you’re comfortable with WP-Pagenavi, install it and modify your code as follows:

<?php
    //You need to handle the 'paged' query var for pagination!
    $paged = (get_query_var('paged'))?get_query_var('paged'):1;
    $loop = new WP_Query(array('post_type' => 'portfolio', 'posts_per_page' => 2, 'paged' => $paged)); 
?>

Replace the following:

<!-- Next/Previous Posts -->
<?php if (function_exists("pagination")) {
    pagination($additional_loop->max_num_pages);
} ?>

With:

<?php
    if(function_exists('wp_pagenavi'))
    { 
        wp_pagenavi(); 
    }
    else
    { ?>
        <div class="navigation">
        <div class="alignleft"><?php next_posts_link('Previous entries') ?></div>
        <div class="alignright"><?php previous_posts_link('Next entries') ?></div>
        </div>
    <?php }
?>

Hope this helps!