Another help source (if you haven't already done so).
Go to your OS Help Menu. In the search field type dvd
Click on all the troubleshooting topics & Support Articles that pertain to your issue.
You can also do the same in Disk Utility. Open same up.
At the bottom left of the window, click on the purple button w/the "?" in the middle.
This will bring up the Help Menu.
Try a different brand. Top forum favorites
Make sure the DVDs are not dirty, smudged and/or scratched.
http://support.apple.com/kb/TS2000 Mac doesn't start up from install disc
http://support.apple.com/kb/HT2801 Apple Computers: Troubleshooting the slot-loading SuperDrive
*** Your drive may need cleaning. Cleaning kits can be purchased from any store that sells CD/DVDs. ***