Need Help with Repairing Startup Volume

OK, heres the problem. I have Onyx and I did the usual diagnostic. It then says that "startup volume needs to be repaired" and that I need to boot into my Mac Os X install DVD and go to Disk Utility. I put in the DVD, press restart, and hold C. While my mac is rebooting it ejects the disk and takes me back to my normal desktop. What did I do wrong? Or what can I do to fix it?