So the problem is that you have set up the button to have a value of yes when if it's a css, it has to have the value of your text.
The alternative is to add in a hidden input field so change your code on the product_info_display.php to
Code:
<!--bof Wishlist button -->
<!--<br class="clearBoth" />
optional depending where the code is added-->
<?php if (UN_MODULE_WISHLISTS_ENABLED) {
if ( $_SESSION['customer_id'] ) {
echo '<input type="hidden" name="wishlist" value="yes" />';
echo '<div id="productWishlistLink" class="buttonRow back">';
echo zen_image_submit(UN_BUTTON_IMAGE_WISHLIST_ADD, UN_BUTTON_WISHLIST_ADD_ALT);
echo '</div>';
} else {
echo '<div>';
echo UN_BOX_WISHLIST_LOGIN_TEXT;
echo '</div>';
}
} else {}
?>
<!--eof Wishlist button -->
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