update lead records - no permission

Hello,

I wrote a script to update a record by ID. The ID is custome field. My script is:

$xmlData = '<Leads>

<FL val="affStatus">'.$newstatus.'</FL>

</row>

</Leads>';

$post=array("id"=>$id,"xmlData"=>$xmlData);

<--curtl-->

I get the message:

You do not have the permission to edit this record or the "id" value you have given is invalid.

How can I correct this?

Thanks