Sending IR (NEC)-Signals with Arduino
It isn't working probably because the output of the Arduino isn't simply an on/off signal.

That library uses a pulse width modulated output on the Arduino to drive an LED.  When the LED is "on" the signal driving it is actually pulsing at a very high rate.

I don't know what all you'd have to change to make that work with a direct wired connection.

Another thing that might becausing problems is that without checking, you can't be sure that the signal level you are sending matches the levels that the IR receiver would normally produce.