You need to add theme support in the functions.php file of your active theme like this:
if ( function_exists( 'add_theme_support' ) ) {
add_theme_support( 'post-thumbnails' );
}
To enable it within a custom post type, you will need to do so when registering your post type with the supports argument, so you’ll need to add something to your already existing code that registers your post type like so (this is an example):
$args = array(
'labels' => $labels,
'public' => true,
'publicly_queryable' => true,
'show_ui' => true,
'show_in_menu' => true,
'query_var' => true,
'rewrite' => true,
'capability_type' => 'post',
'has_archive' => true,
'hierarchical' => false,
'menu_position' => null,
'supports' => array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'comments' )
);
register_post_type('book',$args);
If you see the last line of the $args array, there’s thumbnail in the supports array. That enables it for the custom post type.
See the Codex for more information:
For adding thumbnail support: http://codex.wordpress.org/Post_Thumbnails
For registering post types: http://codex.wordpress.org/Function_Reference/register_post_type
Or more specifically, a complete example with thumbnails added to a custom post type is here: