Q: CD put in; comes right back out...?

The drive accepts discs but they do not mount or are automatically ejected

1. Check the disc's surface for scratches and dirt, since these things can prevent the disc from appearing on the desktop.
2. Reset the Power Manager (PMU) or System Management Controller (SMC).
3. Ensure your drive can read the specific type of disc being used. For example a "combo drive" can read a DVD-R and burn a CD-R however it cannot burn a DVD-R. The utility System Profiler may provide additional information regarding what media your computer supports.
4. If the discs continue to not be recognized, please contact Apple, contact an Apple Authorized Service Provider, or make an appointment with an Apple Retail Store for additional troubleshooting, evaluation, or service.

http://support.apple.com/kb/ht2801