Trading System with Akka
Other than maybe adding some comments for documentation, this seems pretty sane. This:

    System.out.println(String.format("OrderProcessingActor %d %s order received for instrument %s with price %s", order.getOrderId(), order.getOrderType(), order.getInstrument(), order.getPrice()));
    //process order here.
    getSender().tell(String.format("OrderProcessingActor %d %s order processed for instrument %s with price %s", order.getOrderId(), order.getOrderType(), order.getInstrument(), order.getPrice()), getSelf());


is a classic don't-repeat-yourself situation. Make a toString() method on Order:

String toString() {
   return String.format("OrderProcessingActor %d %s order for instrument %s with price %f", orderId, orderType, instrument, price);
}


Then, the prior code can be

    System.out.println(String.format("%s has been received", order));
    //process order here.
    getSender().tell(String.format("%s has been processed", order));