how can we make "other" suggestions if you do not tell us what you have tried or bothered to give any details as to what variety of iMac you have.

1. Reboot and hold down the mouse button until the disk ejects
2. Use the Eject key on the keyboard
3. Right (Control) Click and choose eject from the contextual menu when the DVD is selected
4. Does the Optical Drive make any noise when told to eject?
5. Does the Optical Drive read/play this DVD?
6. Do you have any program open that may be accessing the disk. (Like an open .txt file)