WordPress widget in custom theme

You have to hook your widget registering like that:

add_action("widgets_init", "register_widgets");// Hook WP

/**
 * Register widgets
 */
function register_widgets() {
    require_once "FooWidget.php";
    register_widget("FooWidget");
}

Same for the widget zones

add_action("widgets_init", "register_widgets_zones");

function register_widgets_zones() {
    register_sidebar(array(
        "name" => "Foo Zone Page",
        "id" => "foo-zone-widgets",
        "class" => "panel",
        "before_widget" => '<div id="%1$s" class="widget %2$s">',
        "after_widget" => "</div>",
        "before_title" => "",
        "after_title" => "",
    ));