You have menu two times as parameter, you probably want:

wp_nav_menu( array('theme_location' => 'main', 'menu' => 'menu_center' ));

You have menu two times as parameter, you probably want:

wp_nav_menu( array('theme_location' => 'main', 'menu' => 'menu_center' ));

From your comments I'm getting that you're missing some information about the whole topic, e.g. the difference between the description and the name of a menu. To resolve this you really should just read the according documentation thoroughly:

• WordPress Menu User Guide
• Appearance Menus Screen
• Navigation Menus
  • register_nav_menus()
  • register_nav_menu()