Set the 'showEvent' property to 'click' instead of 'mouseenter', the menubutton doesn't show drop-down menu until the user click the button. Please try this code.
$('#dg').datagrid({
columns: [[
{field:'itemid',title:'Item ID',width:100},
{field:'act',title:'Action',width:100,
formatter: function(value,row,index){
return '<a class="act-btn" rowindex='+index+' href="javascript:;">button</a>';
}
}
]],
onLoadSuccess: function(){
$(this).datagrid('getPanel').find('.act-btn').menubutton({
menu: '#mm',
showEvent: 'click',
onClick: function(){
var index = $(this).attr('rowindex');
$('#dg').datagrid('selectRow', index);
}
})
}
})