MacBook Won't Read Snow Leopard Install DVD?

I tried installing iLife and the DVD was read perfectly fine. However, I realized I needed Snow Leopard in order to install iLife. I tried it and the first time it read it and I accidently hit Not Now when it asked if I wanted to install. Then the disk came out. I've tried putting it in five times and each time it tries to read a few times and just pops out. Did I do something when I hit Not Now or is this a problem with the computer or is it just a bad disk?