I just gave my old iMac to my father-in-law. He's never had a computer with a slot-loading optical drive, and asked me what to do if a disc ever gets stuck in there. I wasn't sure what to tell him since I've never run into that problem.

First hold down the Eject key on the keyboard for a while. If you’re not using a Mac-friendly keyboard with an Eject key, click the disc’s icon on the Desktop and choose File > Eject, or try dragging it to the Trash. If a file on the disc is open, that’ll probably stop it from ejecting, so make sure you’ve quit any applications that might be using files on that disc.

Disk Utility has an Eject button that might work if the Finder won't eject your disc.

If the Finder is still being difficult, Disk Utility should let you eject the disc. And your last resort is to restart the Mac while holding down the mouse button--that should send the disc flying right out at you. If a restart is too inconvenient, open the Terminal, type drutil eject, and press Return.