After browsing the forums for a while, I noticed that the CD key disappearance seems to be limited to those who purchased through Steam for Linux or Mac OS. For those on Linux, and quite possibly Mac OS, if you install Wine and then install Steam through Wine, the CD key appears on the library page for the "Windows" version of Steam. Select the CD Key link, copy the code, and follow the instructions stickied on this forum. There is no need to install the Windows version of Overgrowth through Wine. Windows Steam is all you need to grab the key. Then return to running Overgrowth through the Linux (or Mac OS) version of Steam.