Is there a metal plate closing up the second slot? If so you need to remove the front face plate (The whole thing comes off and is usually held on by some clips or it just pulls of) and then remove the metal plate. Either way the front face need to come off for you to put in your optical drive. Once it is in just put the front face back on and connect power and sata and boot up.
You may have to go into BIOS and enable the sata port it is on. Some of the older dells turn off unused sata ports.