How about this then - forget I brought it up, and just make sure your anodes are connected to the high voltage side of your circiuit and the cathodes to the low voltage side as in the example circuits I posted.
If you have a single LED, the +5v can be an arduino output pin, the transistor is not needed, and the 'bottom' of the series LED/resistor can be ground. High output turns on LED.
Or the +5v can be +5v, and arduino connection can be made at the bottom of the LED/resistor. Low output turns on LED.
I would limit current to 20mA thru the arduino tho.
I'm off to bed....