Here are the steps you will need to follow in order to burn a CD iso image in Mac OSX. (Of course, you will burn a Debian iso imge :).
- insert the empty media (you can allow Finder to handle it or press Ingore, it doesn't matter)
- run Disk Utility (Applications/utilities)
- click on the menu entry Images/Burn...
- select the iso image file and select your burning options
- press Burn
- wait for the disk to burn