Do some error checking, e.g.,
if (mysql_query("INSERT INTO mcards VALUES ('NULL','$MCname','$MCcontent',CURDATE(),'$userID')")) {
header("Location: methods.php?msg=New_Method_Added");
} else {
header("Location: add_mcard.php?msg=dowload_error");
}
Incidentally, note that the URL of a location header should be an absolute URL in order to comply with HTTP/1.1. Also, you should have an exit or die right after sending a location header otherwise the rest of the script will still be executed.