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PDF page 232 (my emphasis):
The final method you need to implement is the most important one, the FromJournal
method, which allows you to map an event into a new event type. In this case, you need to
create a new event from the event that you were provided that contains the important
properties that you are interested in. You start by defining the new event that you'll be using;
in this case, it's a simple class containing a StockIdentifier property and a count. You can
see the class definition in the following example.
You also then define your mapping function by
creating an instance of the new event from the old event.

The text mentions a class definition that doesn't exist in the code sample:
public IEventSequence FromJournal(object evt, string manifest)
  if(evt is Events.V1.ItemAdded)
    var oldEvt = (Events.V1.ItemAdded)evt;
    var newEvt = new Events.V2.ItemAdded(new
    return EventSequence.Single(newEvt);
  return EventSequence.Single(evt);