function requestMe() {
  var req = opensocial.newDataRequest();
  req.add(req.newFetchPersonRequest(
            opensocial.DataRequest.PersonId.VIEWER),
          "viewer");
  req.send(handleRequestMe);
};

function handleRequestMe(data) {
  var viewer = data.get("viewer");
  if (viewer.hadError()) {
    <em>//Handle error using viewer.getError()...</em>
    return;
  }

  <em>//No error. Do something with viewer.getData()...</em>
}

I have two doubts:
What does that add() do there?

And when I am supposed to write the definition for send() I need to using the argument as callback. Now how do I write code that does a callback to the agrument?