$sql = "INSERT INTO mainnews (title,desc,body,added,userid)
VALUES ('$title', '$desc', '$body', '$added', '$userid')";
$result = mysql_query($sql);
//use this debug code:
$err=mysql_err($result);
$errno=mysql_errno($result);
echo "$sql<BR>$err<BR>$errno";
//
You may be having trouble adding the date, which requires a different format than M d Y
try using the SQL current_date() function instead, which always works:
$sql = "INSERT INTO mainnews (title,desc,body,added,userid)
VALUES ('$title', '$desc', '$body', current_date(), '$userid')";