OS X doesn’t have an Erase Disk command at the desktop. When you want to erase a disk (such as a DVD-RW disc), you’re supposed to use Disk Utility, which sits in your Applications→Utilities folder. This is the same program you use to erase, repair, or subdivide (partition) a hard drive.
Once you’ve opened Disk Utility, click the name of the disk (in the left-side list), click the Erase tab, and then click the Erase button.
You won’t be able to do so, though, if:
The disk is a standard CD-ROM, DVD, a previously recorded CD-R disc, or a disk elsewhere on the network.
You’re trying to erase the startup disk. You can’t wipe out the disk that contains the currently running System folder any more than you can paint the floor directly beneath your feet. (To erase your built-in hard drive, for example, you must start up from the OS X DVD.)