You can't from the desktop Day. You need to reboot your PC.
The very first screen you come to is called the post screen. For example:
You can see on the example above, that the PC has picked up an ASUS CD-S520A. You should have a similar page unless you have an e-machines logo instead. If so, you'll need to press F2 or [DEL] to get into BIOS. Every BIOS is different, so it would be very difficult to talk you through the exact process, however you will need to look for your Primary Master, Secondary Master, Primary Slave and Secondary Slave.
For example:
In your Advanced tab, you will also see a setting to turn off the e-machines logo so that you can see if your CD-ROM is being displayed at this point. If you have problems, please can you describe in detail what you see.