If a data item has 'selected' property set to true, it will be selected when loaded successfully. All you need to do is to set 'selected' property for the specified item from your returned json data. Look at the following data, the 'Ruby' item will be selected since its 'selected' property is set to true.
[{
"value":1,
"text":"Java"
},{
"value":2,
"text":"C#"
},{
"value":3,
"text":"Ruby",
"selected":true
}]