Transmitter:
Serial.flush();
You never use Serial.begin() in the transmitter, so why do you flush it? Note: .flush() really does nothing useful. It is extraordinarily rare to see that in any program.
That easy transfer seems like a very old library. 2011 is almost the dark ages of Arduino programming. It should still work but I'm suspicious that its detection of the Arduino version number may not still be valid.
Receiver:
delay(100);
Remove this from the receiver. It's only going to make you miss data.
But you got part of the message, so something is working. That's a great clue that will help discover what isn't working. Maybe try changing the values in the sender? Make it send millis()/100 or something.