Is there a DVD in the drive in the drive you want to eject?
MacBooks use slot drives meaning there is no tray at all, you just push the CD/DVD (gently) into the slot til the drive 'grabs' it and pulls it in (HT1360).
In case you need to eject a disc which doesn't react to the usual means, the following may help (taking more or less verbatim from Apple):
Ejecting a CD or DVD when all else fails
- First, quit all applications that are using files on the disc. Then try one of the following alternate methods
- Open Disk Utility (in the Utilities folder of your Applications folder), select the disc, and choose File > Eject.
- Choose Apple menu > Restart and hold down the mouse or trackpad button until the disc ejects.
- Reboot the computer and keep the mouse/trackpad pressed