var lookupItem = new Array; 
lookupItem = crmForm.originatingleadid.DataValue;

if (lookupItem != null) 
{ 
crmForm.all.IFRAME_LeadNotes.src="/_controls/notes/notesdata.aspx?id="+  lookupItem[0].id +  "&ParentEntity=3&EnableInlineEdit=false&EnableInsert=false"; 
} 
else 
{ 
crmForm.all.IFRAME_LeadNotes.src="about:blank"; 
}