well first you have to order a drive, after quick google searching it seems your dell uses a SATA dvd drive interface
sata dvd driveshttp://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&N=-1&isNodeId=1&Description=sata+dvd&x=0&y=0
there are two connectors on the back of the drive
SATA (serial advanced technology attachment) connects to the red cable (data cable), its not as wide comparing to the power cable that runs right next to it.
look at your power supply cables and try to find a thin and relatively wide connector to get the power to the drive
if your power supply is really crappy it might not have the power cables, in that case you will need a MOLEX adapter like shown belowhttp://www.bestpricecables.com/images/thumbnails/10/200/POWER-ATA-2PY.gif
make sure to have your computer disconnected from the wall socked before opening the case