Please try this code:
$('#dg').datagrid({
columns: [[
{field:'name',title:'Name',width:100},
{field:'action',title:'Action',width:150,align:'center',
formatter: function(value){
return '<span class="action">'+value+'</span>';
}
}
]],
data: data,
onLoadSuccess: function(){
$(this).datagrid('getPanel').find('.action').menubutton({
menu: '#actionMenu'
});
}
});