Installing WP-PageNavi for multi-part posts

Hey guys I’m just trying to find out what’s involved when trying to install WP-PageNavi for multi part posts when the line
<?php wp_link_pages( ... ); ?>
is not present in any file. As such I have no idea where to put this line <?php wp_pagenavi( array( 'type' => 'multipart' ) ); ?> The theme has pagination on the home page and archives but nothing at all for multi part posts. Is it still possible to install it and get post pagination working? If so how?

Thanks for any help!

http://wordpress.org/extend/plugins/wp-pagenavi/