for Add:

jQuery("#griddetail").jqGrid("editGridRow","new");

for Edit:

jQuery("#griddetail").jqGrid("editGridRow","idno"); //replace "new" with record id no eg "2"

for Delete:

jQuery("#griddetail").jqGrid("delGridRow","idno");

Eg of use in Delete button:

$("#buttDelete","#t_griddetail").click(function(){
    var s;
    s = jQuery("#griddetail").jqGrid('getGridParam','selrow'); //get the selected record no
    if (s==null){
       alert("Please select a record.");
    }else{
       jQuery("#griddetail").jqGrid("delGridRow",s,{reloadAfterSubmit:true,jqModal:false, closeOnEscape:true});
    }
});

hope this help someone else

Better still if someone can tell me if dynamically enable/disable the Edit button in the navGrid toolbar is possible, and if so how to go about it.

I did this:

toolbar: [true,"top"]

And I check my row record with this:

 onSelectRow: function(ids) {
   var ffs = $('#griddetail').jqGrid('getCell', ids, "Confirmed");
   if (ffs==1){
    $("#buttEdit","#t_griddetail").attr("disabled",true); //if record is confirmed - do not allow editing
   }else{
    $("#buttEdit","#t_griddetail").attr("disabled",false);
   }
 }

Then added this:

$("#t_griddetail").append("<input type='button' value=' Edit ' disabled=true id='buttEdit' style='height:20px;font-size:-3'/>&nbsp;&nbsp;&nbsp;&nbsp;");

Now what should I do if I wanted to activate the Form for Editing?

$("#buttEdit","#t_griddetail").click(function(){
 
});

Thanks.