I’ve done this by positioning the wishlist manually and changing the code to:
<?php if( is_user_logged_in() ) {
echo wpsc_the_wishlist();
} else {
echo '<button type="button" class="button"><a href="http://yoursite.com/products/your-account/?wishlist=true">Add to Wishlist</a>'; } ?>Just make the url point to your wishlist page which should prompt them to login, or you could make it a fancy popup message etc.
