Add Menu Support

Backup current menu code
When you add menu support to a Wordpress theme, you need to specify some fallback code in case the user has not setup a menu in the Wordpress interface yet. The way to do this is to open up the header.php file in your theme and look for a line of code that represents how the menu is currently being written. In most cases the code will look like this:
 * 

Go ahead and copy this code so you can add it as fallback in the next step

Adding menu support to the theme
The code we're going to add to enable menus needs to go in the functions.php file within the theme folder. Go ahead and open it up and put your cursor a few lines underneath  'The Main Menu',
 * }
 * add_action( 'init', 'mytheme_addmenus' );
 * function mytheme_nav {
 * if ( function_exists( 'wp_nav_menu' ) )
 * wp_nav_menu( :'menu=main_nav&menu_id=pagetabs&fallback_cb=mytheme_nav_fallback' );
 * else
 * mytheme_nav_fallback;
 * }
 * function mytheme_nav_fallback {
 * wp_list_pages('title_li=');
 * }
 * mytheme_nav_fallback;
 * }
 * function mytheme_nav_fallback {
 * wp_list_pages('title_li=');
 * }
 * }

Let's go through this code so we understand what's happening.
 * 1) The first function is defining a new menu, which they are calling "The Main Menu".
 * 2) The next function initializes that menu in the theme, basically activating the Menu support in the theme and providing the Main Menu as the default menu available. Note: This alone does not activate The Main Menu for the theme, because the menu would be empty by default. The user has to enable that menu to be used in place of the fallback menu within the Menus section of Wordpress.
 * 3) Our next function says "If the user has enabled their menu on the theme, put this line of code in the header to display that menu. If not, use the code in the next function instead."
 * 4) Our last function provides the code that will be used if no menu is activated yet.

Adding Fallback Code into the Menu
Now it's time to take that code you grabbed from the header and put it into the last function mytheme_nav_fallback. In most cases this line will be a variation of wp_list_pages with arguments.

Calling our new menu code in the Theme
The last step of the process is to go back into header.php and update the menu to use our new code we added in functions.php. Since the code in functions.php handles all the menu support and fallbacks, all we have to do is call one function for it to work. Replace the original menu code (again, probably wp_list_pages(arguments...);) with the following code: 

All Done
Menu support should now be active on the theme. To test this activate The Main Menu in the Menus under Appearance and add some items.