The "official" way to erase a CD-RW in Mac OS X is to use the program /Applications/Utilities/Disk Utility. Using this program is less than intuitive, and an easy "one-click" method is available:
The CD-Erase Program
The program has two options, to erase the data "Quickly" or "Completely". The quick option takes approximately one minute and the complete option more like 30 minutes.
The program shows a "barber-pole" style progress bar while it is erasing the disk.
Once the disk has been erased, the program returns to its initial "erase disk" screen, at which point you can eject the disk to insert another or press cancel to leave the program.
Options for this page: