header menu customization

I’ve just added a header menu to my layout as you can see here
I’m trying to style it but I’m not really sure how.
This is what I have right now in header.php. But the php wp_nav_menu is not accepting the div id headermenu

<div id="page">
<div id="headermenu">
<?php wp_nav_menu( array( 'theme_location' => 'header-menu' ) ); ?>
<div id="header">
	<div id="headerimg">
	</div>
</div>
</div>