I just got a used Sega CD from a fellow college student with the warning that it might not actually work. It turns on and seems normal, but how do I open the disk drawer? And what does this "lock" button on the left side do?
You have to open the drawer with your control pad from the menu screen. Turn on the system let it check for a disc then press the start (or the C) button this will bring up the menu screen from here select the "Eject" button, voila! it's open!
The Lock Switch on the side is to hold the mixing cable from the Genesis' headphone jack out of the way of the CD drawer.
If you don't use the mixing cable the lock isn't used.