AddOnLoad() with re-direct

I've re-visited this after deciding to purchase phpr and now I'm having problems that I'm sure I didn't have before with the demo download. The version I have is 4.0 build 265.

Basically I want to redirect ADD if displaying Master with Detail. I'm getting a 'Cannot modify headers error message using the header command and if I try the script solution as mentioned in a prior topic regarding listonload , I get re-directed to the menu with the message 'Your session has expired. Please login again' .
error using header code: Cannot modify header information - headers already sent by

(output started at C:\Inetpub\wwwroot\ShotStore\templates_c\%%46^46C^46C9D2DD%%jobs_add.htm.php:8)
Code with header ..... commented out :
function addonload

//** Redirect to another page ****

if (isset( $_SESSION['jobs_masterkey1'] )) {

//Coming from Master detail so make cust read only

//header("Location: jobsro_add.php");

?>

<script>

window.location='./jobsro_add.php';

</script>

<?php

exit();

}
Any help appreciated.