please help with dvd problem I am desperate

Ok so I bought a refurb MacBook and when I got it home it wouldn't read dvds. I took it back and they gave me a new one. That one had some problems with the screen so I took it back but didn't try the dvd. They gave me a 3rd MacBook and showed me that the dvd worked by putting it a Mac OS. When I got it home I put in a dvd and it didn't work. I tried 20 other dvds and only the Notion3 dvds worked.
I then tried burning a dvd and it burned one fine but when I put it back in to play I got "the disk you inserted was not readable by this computer".
It worked fine on my PC when I tried it.
Any Ideas?
Thanks

What program are you using to burn with, and what brand of discs?
Certain brands, Maxell in particular, just will not work for some reason.

I just used idvd to burn I think and I am using Maxell actually. But why would it burn it fine but not read the same disk it burned. It won't read actual real movies either so I don't know whats going on.

Semi-dirty lens or an optical drive on it's way out.

Take a can of compressed air and give it about 3 quick bursts in different directions inside the slot - after you've shut the machine down and wait for a minute to turn it back on.