Lift the side of the optical drive closest to you, then slide the drive towards you, and up and out of the computer.

Follow these steps to reinstall the optical drive:

First, slide its side nearest to the rear of the Macbook under the edge of the rear frame to the left of the hinge, while also sliding the optical drive's mounting tab at its upper left corner under the cables at this location.

Lower the drive partially into the lower housing. Keep the hard drive cable away from the optical drive bay.

Before dropping the drive fully in place, use a spudger to push forward (towards the front of the drive) on the screw hole in the drive's mounting tab.

Push forward the slider, which runs along the far side of the drive, to insert the end of this slider into a small channel in the lower case's frame. This helps hold the drive in place.

When you push the slider forward, if the screw hole in the drive's mounting tab doesn't line up over its threaded brass insert in the lower case, the front end of the slider hasn't fully inserted into its channel in the frame. Keep trying, but if the slider won't move further to the right, remove the drive to see if the end of the slider is bent.

If the front end of the slider isn't bent, it may be slightly too long. Use a pair of fine cutters to clip off the narrower portion of the end of the slider. The optical drive will still remain firmly in place.