On the outside of a Dell desktop, where are the

Below is a link to the users manual on the dell web site. It shows how to open the case and access the DVD drive area and replace or install a second DVD. You will need a med size phillips head screwdriver, perhaps a med size standard screwdriver to help depress tabs etc. It's a pretty straight forward job just make sure you disconnect all devices from the box, and unplug it to avoid static shock.

http://support.dell.com/support/edocs/systems/dim4600/en/4600i/index.htm