Well, it's hard to diagnose something like this at a distance, but here are some more questions, and a few things to try (not necessarily in any particular order):

1) Does the drive reject all discs, or only DVDs? Try some factory-pressed CDs, as well as some burned ones. If it consistently rejects DVDs but not CD-Rs, that's a big clue (and one that usually points to a bad laser).

2) Try reinstalling the CD/DVD extension.

3) If it's a tray-loading drive, inspect the lens. If it looks dirty, gently clean it. Beware -- there is a thin coating that rubs off if you get too eager.

4) If it's a slot-load drive, it's a lot harder to inspect the lens. If you get desperate enough, though, you may want to give it a go. If so, post back, and I (or someone else) will help walk you through the procedure. Otherwise, leave it be for now.

5) It's also possible that a connector has worked loose in transit along the way. Inspect/reseat the drive connectors at both ends. Maybe you'll get lucky.

These are just a few quick thoughts. Post back with some results, and we'll see if there's anything else you can try.