Either the disk is faulty or your Superdrive is.
Try cleaning the Superdrive with a propriatory CD/DVD lens cleaner that uses tiny brushes. All computer stores sell them.
Next, do this:
Resetting the SMC (System Management Controller) on Intel-based Macs:
http://support.apple.com/kb/ht3964 and
http://support.apple.com/kb/HT1237?viewlocale=en_US
Resetting your Mac's PRAM and NVRAM:
http://support.apple.com/kb/ht1379
Reboot, repair permissions, and try again.