How to configure WordPress to be able to use tag inside posts?

Add the following to your theme functions.php:

function fb_change_mce_options($initArray) {
    $ext="script[charset|defer|language|src|type]";

    if ( isset( $initArray['extended_valid_elements'] ) ) {
        $initArray['extended_valid_elements'] .= ',' . $ext;
    } else {
        $initArray['extended_valid_elements'] = $ext;
    }

    return $initArray;
}
add_filter('tiny_mce_before_init', 'fb_change_mce_options');