One of the things that may confuse you is as follows:
CD/DVD drives normally connect to the computer with a cable called an IDE cable. An IDE cable looks like this:
http://www.logicsupply.com/images/photos/cable s/44pinIDE_big.jpg
One end goes in the CD/DVD drive, the other goes in the Motherboard of the PC.
If you want to add an extra CD/DVD drive you have to use an IDE cable like this (notice it has an extra connector in the middle)
https://www.ccity.ie/site/components/com_virtuemart/shop_image/product/a347eff2b0dc243a933aee01cc9a14ae.jpg
You connect one end to the motherboard, the other end to ONE CD/DVD drive, and the middle connector to the OTHER CD/DVD drive.
Now I dont know if your Dell has the first type of IDE cable or the second.
If it has the first type you would need to buy a new IDE cable with the extra connector, take the old IDE cable out and put the new cable in.
Not difficult, but something you need to be aware of.