That iMac should have firewire, get an external, one of these.

Then download Patchburn, the links are on that page. You can get the same model for $7 US less, but this one includes DragonBurn software to burn data discs. It says Finder burning should work after Patchburn, but I installed the Pioneer 116 mechanism in a tower this week and Finder burns didn't work, they needed a third party solution, like DragonBurn, or Toast.