Crossover downloads Internet Explorer 6's installer from the Microsoft Download Center. As of this week, Internet Explorer 6 has been removed from the download center. New installation of Internet Explorer 6 are no longer possible. Even if you already have the installer file for IE6, that file will "phone home" during the installation process, and the files that it needs to pull down from the Microsoft Download center are gone.
If you have an existing install of Internet Explorer 6, please use Crossover's 'Manage Bottles>Advanced>Make Archive' function to create an archive of your existing install for safe keeping in case you should need to delete your current bottle and re-install. A restored archive should work, but a re-install will not.