I agree with BenfordAV that the DTV boxes have finicky IR control - but in my experience they can be made to work reliably.

You will need to experiment, but one trick which has worked for me is to cut a piece of black electrical tape large enough to cover the IR sensor on the DTV box. Poke a pinhole in the center of the piece of tape and place it over the sensor. You may need to adjust the pinhole size to get reliable control.

For a more finished look, take one of the Xantech IR emitter shield covers (the covers used to prevent iR "leakage" from emitters) and cut off the flat portion. Use the flat portion of plastic instead of electrical tape.

Whenever possible, I use RS232 control for these boxes. Unfortunately sometimes there's no budget for an RS232 capable processor.