Assign the 'id' property for each button.
toolbar: [{
id: 'btn-add',
text:'Add',
iconCls:'icon-add',
handler:function(){alert('add')}
},{
id: 'btn-cut',
text:'Cut',
iconCls:'icon-cut',
handler:function(){alert('cut')}
}]
And then get the linkbutton by the 'id' property.
$('#btn-add').linkbutton('enable');