One straightened paper clip or similar tool.
1. Place the iMac on its left side (when facing the computer) on a non-abrasive surface to prevent scratching the case.
2. Hold the CD-ROM tray open such that the bottom of the tray is visible. Be careful not to touch the lens on the other side. See Figure 1.
Figure 1 Eject Mechanism
3. Move the exposed tab of the eject mechanism toward the CD bezel as shown in Figure 2. Continue until it clicks into position. The eject mechanism is now properly engaged.
Figure 2 Eject Mechanism Reset
As the eject mechanism is engaged, the small, forward portion of the mechanism becomes exposed as shown in Figure 3.
Figure 3 Eject Mechanism Gap
4. If the eject mechanism does not engage, ensure that the small gap between the tray bezel and the metal housing is clear of obstructions. See Figure 4.
Figure 4 Engaged eject mechanism and small gap
If the CD-ROM tray still does not latch closed, the iMac requires service. An Apple Authorized Service Provider can remove the drive and repeat the procedure, or replace the drive if that fails.