Here is a link to a previous question that should help you.

It says to turn the computer on while holding down the mouse button.

Some people use a two sided tape technique to get it out.

Others try to eject the disc with the computer on it's side and let gravity help.

Only the earliest iBooks had a manual eject on the drive.

Still others have found the drive bezel bent and used a wide flat head screwdriver to "open up" the slot more to make room for the disc to come out.

Good luck, you'll get it.

Frank