You can, but you have to get your hands dirty with code. First and foremost, backup your files before you start editing them!
Okay? Cool. In your functions file add this line somewhere:
register_nav_menu( 'primary', __( 'Primary Menu', 'arcsin' ) );
Got it? Now in your header.php file look for the line that has:
<?php sm_mainnav(); ?>
And replace it with this:
<?php wp_nav_menu('menu_location=primary&menu_id=&menu_class=tabbed'); ?>
Let me know how it worked out. When you put those in, you can now use the WordPress Custom Menu system found under Appearance. Note: You have to be using WordPress 3.0+ to do this, but you should already be running WP 3.2.1