You have to open up the computer on the side, then remove the plastic spacer from the inside. If you push on it from the front, it's going to break.
If you're using two CD drives, before installing the new CD-RW drive, you'll have to configure it to be a secondary drive. Search your installation manual for 'Master / Slave', so you can find out what to set the plastic jumper pin on the back of the drive at, for the 'Slave' setting.
The hard drive cable that connects both drives to your computer will have three plugs...one end connects to your motherboard, the other end connects to the primary drive, and the middle plug connects to the secondary (CD-RW) drive. Don't forget to plug in the power cable as well (it has a white plug, with small red/yellow/black wires running into it)
If the CD-RW drive is going to be your only drive, then it should be straightforward. Just plug in the cables from your old drive into the new drive.