Use up/down arrow should able to select the menu items.
Just try the attached program below.

This cause an accessibility bug in evolution, see
http://bugzilla.ximian.com/show_bug.cgi?id=41633



#include <gtk/gtk.h>

int
main(int argc, char *argv[])
{
        GtkWidget * window;
        GtkWidget * opt;
        GtkWidget * menu;
        GtkWidget * menuitem;


        gtk_init(&argc, &argv);

        window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
        opt = gtk_option_menu_new();

        menu = gtk_menu_new();
        menuitem = gtk_menu_item_new_with_label("first");
        gtk_menu_append(menu, menuitem);

        menuitem = gtk_menu_item_new_with_label("second");
        gtk_menu_append(menu, menuitem);

        gtk_option_menu_set_menu(opt, menu);

        gtk_container_add(GTK_CONTAINER(window),  opt);
        gtk_widget_show_all(window);

        gtk_main();

        return 0;
}