Q: using a CD/DVD

You have to buy an external optical drive.

The reason the iMac is so thin is that Apple did away with the CD/DVD optical drive.

You must purchase an external USB optical drive, now.

Apple makes an external optical drive, it only works with newer Macs that no longer have a bulit-in Super Drive.

OR you can purchase other third party optical drives.
Not all optical drives are Mac compatible.
Most Mac compatible drives you will only find to be purchased online.
Here are two on Amazon.com that are cheap and Mac compatible

http://www.amazon.com/LG-GP10NB20-Portable-External-Drive/dp/B004K2Z4MS/ref=sr_1 _3?ie=UTF8&qid=1398883289&sr=8-3&keywords=Mac+compatible+external+LG+optical+dr i ve

http://www.amazon.com/LG-Electronics-Portable-External-GP40NB40/dp/B009DS6OFU/re f=sr_1_7?ie=UTF8&qid=1398883289&sr=8-7&keywords=Mac+compatible+external+LG+opti c al+drive


Here's another source for Mac compatible optical drives. These are better quality optical drives.
These are also tray loading drives.
Tray loading optical drives are more reliable, plus they are capable of playing those smaller mini sized discs.

http://eshop.macsales.com/item/OWC/VLU2SD24XS/

http://eshop.macsales.com/item/OWC/MR3F82SD24S/

Good Luck!