Solution:
Had the exact same issue, the answer annoyingly is that you can only output Realmags currency changer on pages with a product in them. A very hacky solution is to include a product in your footer and hide it with CSS.
footer.php
<?php echo '<div class="hidden">'.do_shortcode('[product id="<ID OF YOUR PRODUCT HERE>"]').'</div>'; ?>
styles.css
.hidden {
display:none;
}