[Plugin: RS Simple Category Selector] How to print the selected main category on the page?

Hi!
Is there a way to print the main category, selected among others on the edit post page editor?
I’m trying the following code:

<?php
                $category = rs_get_main_category_id( $post_id );
                echo $category;
                ?>

But it’s not working, even if I set a number for the array, like this:

<?php
                $category = rs_get_main_category_id( $post_id );
                echo $category[0];
                ?>

Thanx.

http://wordpress.org/extend/plugins/rs-simple-category-selector/