If you have disconnected or turned off your power supply before installing the DVD writer (and you should have :P), then maybe you just forgot to turn it on/plug it back in :roll:
Also if it was not a SATA drive but an older PATA drive (ie. you plugged it in with a wide gray ribbon), make sure you have set the master/slave jumpers properly on the drives connected to that ribbon.
Simplest solutions first, guys. Why even mention RAM or graphic cards when it was only adding an optical drive...