why won't my macbook read the dvd i put in?

To remove a CD / DVD that is unmounted:
Restart and hold the click button for 20 seconds. The disc will eject. {He asked "What is the click button?" I answered "The button or area of the trackpad you press to click."}

MingGuang: All recent Apple optical drives DO READ DVD+R.

The DVD may have surface damage. Try that spray-on goo from a music or movie shop that can repair lightly-damaged discs.

From the link below:
"Maximum read: 8x DVD-R, DVD+R, DVD-ROM; 6x DVD-ROM (double layer DVD-9), DVD-R DL (double layer), DVD+R DL (double layer), DVD-RW, and DVD+RW; 24x CD"

Try if it reads other dvds. It would not read DVD+R because the drive does not support so.
Also check if the discs are not damaged.