on the back of your cdrom there should be three or four things (depending on whether you have an audio connector or not). These things are, your power connector, your IDE connector, and some small pins (should be about 10) arranged in a 5x2 fashion. on these pins there should be a jumper. somewhere on your cd-rom there should be a diagram or an indication of where you need to have the jumper to set it to master with slave present, slave, master, and cable select. Just put it on master and your set.
By the way, if you ahve your cdrom on the same IDE channel as your hard disk, you dont want to have your CD-rom set as master. you want it as slave.