Get theme to accept wordpress 3 drop down menu

easiest way I found, is to copy straight from the twentyten theme.

This part into functions.php

// THIS THEME USES wp_nav_menu() IN ONE LOCATION FOR CUSTOM MENU.
if ( function_exists( 'register_nav_menus' ) ) {
	register_nav_menus(
		array(
		  'primary' => 'Primary',
		)
	);
}

and this part where you want the menu (probably header.php)

<div id="menuWrap">
<div id="access">
			  <?php /*  Allow screen readers / text browsers to skip the navigation menu and get right to the good stuff */ ?>
				<div class="skip-link screen-reader-text"><a href="#content" title="<?php esc_attr_e( 'Skip to content', 'twentyten' ); ?>"><?php _e( 'Skip to content', 'twentyten' ); ?></a></div>
				<?php /* Our navigation menu.  If one isn't filled out, wp_nav_menu falls back to wp_page_menu.  The menu assiged to the primary position is the one used.  If none is assigned, the menu with the lowest ID is used.  */ ?>
				<?php wp_nav_menu( array( 'container_class' => 'menu-header', 'theme_location' => 'primary' ) ); ?>
			</div><!-- #access -->
</div>

then you can just grab the block of css from twentyten. Its a bit long to post here, but it starts like this

/* =Menu
————————————————————– */

drop it all in the right places, and it should work. You can then tweak the css to fit your theme

ANywhere in functions.php inside of the <?php and ?> tags